The reaction of the lye with aluminum produces sodium aluminate. It is a light colored solid that is left at the place where the reaction took place. It dissolves in water and forms a caustic similar to lye - it will burn skin, and if ingested can burn mouth, throat, esophagus, etc. Soaking in water should dissolve it, and it can also be removed with a wire brush, better, a wire wheel on a drill. But it isn't safe - get rid of it or get rid of the pan.

If the tile was not sealed when installed, there is little chance of getting all the grease out of the tile/grout. You might clean with a degreaser intended for garage floors.

Any spray on general purpose cleaner....

So, to be clear, I wasn't really talking about the drywall - I was talking about the backer-board behind the tile. That should be 1/2" cement board or something similar. You won't be able to feel it, you will need a visual. If it's a bad install, two things that can happen: 1. There was no waterproofing and so the water seeps through the grout and such and into the backer board. 2. The backer board sits too far down on the tub, without an air-gap. And because of that it is wicking up water from the edge of the backerboard. You will only be able to determine that (without taking the tile off) by getting a visual of the back side of the backer board. If the bottom edge of the backerboard looks dark from moisture, it will have to be redone. ____________________ Since you say that the door is properly caulked, the other place I'd look is the junction between the tile and the tub. That appears to be grouted when it should have been caulked. Anywhere there is a change in plane it should be caulked, not grouted. To do a better job, fill the tub with water first so that there's weight in the tub before you caulk. Yes, water sitting on that tub/tile edge over time can cause moisture to wick horizontally through the grout and cause the issue you see on the drywall. Caulk the tub/tile joint to prevent water that sits there from being drawn into the grout.