The exact environment the pipe are exposed to determines if the copper corrosion is green or black. Green is usually salt corrosion (and there are multiple salts besides plain old NaCl). Black is often associated with sulfur based corrosion. The outer colored layer is usually very thin and does not affect the pipes strength or life, unless something like salt water is dripping on the pipe and running off with now dissolved copper.

From your description it appears that you have valve on the end of a galvanized iron pipe and the copper pipe is attached to that. In all probability the copper pipe has now become fused to the iron pipe by electrolysis. If the valve is open and if it does not leak I would ignore that valve and move on down on the copper line. Cut the copper and install a new valve.

Given the restrictions that you described, I would be inclined to decide if an alternate route for the pipe is available if the garage should be built and the pipe should fail at some point thereafter. Will adequate access to the well itself be available? If the answers are yes, I would just not mention the pipe at all-- and build the garage. It is unlikely that any official record exists of the location of the pipe and even if it does, it is unlikely that any official will bother to look for it.

I have not previously entered into this discussion because "Homebound" had already given the correct answers, but let me add a bit of information to what Homebound mentioned. Sparklingwater mentioned that the leak is at the base of the stem in the angle stop valve. He/she also asked if there is a way to lubricate the stem to make it easier to turn? I suspect that the angle stop had previously been turned off and the HVAC tech turned it on to check out the humidifier and while doing so the leak was noticed. (see attached photo). If you look at the valve you will note a nut on the top of the valve where the handle stem enters the valve body. Under that nut there is a packing that is made out of graphite, teflon or perhaps neoprene that surrounds the stem shaft and as the nut is tightened the packing is squeezed against the shaft to form a water tight seal. When trying to rotate the handle on an old valve you can slightly loosen that nut (generally about a 1/4 turn) and it will loosen the shaft making it easier to turn the handle and once you get the handle to the desired location, either full open or full closed, you can then tighten that nut again to stop any leak that might form. If you cannot stop the leak by tightening that nut you must then replace the valve. (for larger valves we can replace the packing but since angle stop valves are only about $3 for the complete valve, don't expect your local hardware to carry replacement parts for it). That valve is on the hot water system so if you have to change the valve you don't have to drain the whole house, just the hot water system until it is drained below that valve. To drain it down sufficiently you could turn the cold water supply to the water heater off, then if you have a laundry connection of utility sink that is below that valve you could open a hot water faucet on the floor above to vent the line and open the hot water valve at tne sink or laundry and let it run until the water stops. If you don't have a hot water faucett below that level you will then have to turn the gas off to the water heater and drain the water heater until the water is below that valve. That valve is mounted on the vertical copper stub by means of a compression fitting on the bottom end of the valve. Unscrew the compression nut and let it drop down to the tee, then twist the valve a bit with a wrench to loosen it, and it will lift right off. You will then see a brass ferrule surrounding the pipe about 1/2" from the end. In many cases you can remove the compression nut and ferrule from a new valve, then set the new valve down in place and tighten the existing compression nut to lock the new valve on and seal the joint. If it won't seal using the existing compression nut and ferrule you then unscrew the existing nut again and cut the copper stub just below the existing ferrule. You will find a mini tubing cutter will work the best here, but also keep in mind that you have to cut is high enough on the stub to allow the valve to set down tight without hitting the tee. Once you have the pipe cut. clean the pipe with an emery cloth, then put the new compression nut on the pipe, open threaded side up, then slide the new ferrule on the pipe and slip it down about 3/8". Now place the new valve on the stub and push it down tight, the ferrule will slide down to the correct position as you push the valve down. Now slide the compression nut up and screw it on the bottom of the new valve and while holding the valve body with one wrench, thorougly tighten the compression nut with another wrench to lock it in place. Now in regards to that corroded elbow. First off, I would be curious to know what they are draining with that line? I WOULD NOT be worried about that corrosion because it has nothing to do with the water in the line. That corosion resulted from improper installation of the elbow. That white corrosion is a result of excess flux left on the line when they soldered it, however by now the flux has long since dried out and it is no longer a problem. After reviewing the photos I beleive the entire installation was done as a DIY project. Notice the elbow on the left end of the horizontal line where the angle stop is located. That dark coloration on the outside of the curve indicates that they got the pipe too hot while soldering. The same is evident on the side inlet and pipe on the tee right above it, as well as the excess solder on the pipe directly below the tee. Those are all indications of a very amateur soldering job. Next look at the top of the water heater. The copper lines are connected directly to the water heater, but code has required a dielectric coupling since the late 1970's and code requires a union on both water lines since the mid 80's. Now I would not recommend bothering to add the dielectric nipples or unions now, but keep in mind that the next time you have to replace the water heater, make sure they are installed. For the moment I believe if you just open or close that angle stop, then tighten the stem packing nut a bit you will be fine and their is no reason to spend your money calling a plumber.