What kind of grass are you growing if it is bermuda it's not a big deal, but if your growing a cool season grass with seed you will need to use some activated charcoal.

First contact your local office of the Clemson USDA Cooperative Extension Service and the can help identify that and can also help you get a good, reliable soil test which might do as much to control this as anything else. Many "weeds" grow in soils that are not good soils for turf grass and turf grass most often needs a good, healthy soil or it will not grow well. At the same time dig in with these simple soil tests, Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top. 2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up. 3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart. 4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell. 5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy. to learn more about the soil you have and what yo need to do to make that soil into something that will grow a good, healthy turf. Here is a link that might be useful: Clemson CES

If you live in the south and want bermuda or St Augustine, we can help you get to a nice lawn in a few months. If you live in the north and want something like rye, fescue, and/or Kentucky bluegrass, I would suggest you hang onto that thought until after summer as suggested by someone in your past. In the mean time practice proper watering, mowing, and fertilizing and you will have the best looking weeds on the block. That's what I'm currently doing at my new home. Because I'm satisfied with the look of green, even though it is green weeds, this is a very low stress approach. Here's a picture of my weeds from last July. Proper watering means deep an infrequent. Deep means 1 inch all at one time; not spread out over several days. Infrequent means to adjust frequency for the temperature. When it's hotter you deep water more often. With temps in the 90s water once per week. With temps in the 80s water once every 2 weeks. With temps in the 70s water once every 3 weeks. With temps below 70 water once a month. This works in Phoenix and in Vermont, so wherever you are it will work. Mulch mow most grasses at the highest notch on the mower. The only grasses that get mowed at the lowest level are bermuda, centipede, and creeping bentgrass. Fertilize once in the late spring and twice in the fall. Late spring is Memorial Day. Early fall is Labor Day and late fall is around Thanksgiving. That's about all there is to it.

It is very blurry but it is obviously not a turf grass so yes, it is a weed :-) What precisely I can't say unless you can provide a clearer photo and one of just the mystery plant (leaf, stem and flowers).