it's definitely not nutsedge, which has a different and distinctive look. first question to ask yourself:is it the exact same grass as the rest of your lawn? if it is, then extra water or fertilizer in that area is producing extra fast growth. If it's definitely a different plant, then you have a weed grass in there. I've had that. Big mess. Hard to get rid of. Good luck

How long since spreading the seed has it been? Germination of the seeds listed is in the 7 to 21 day range so if it is sooner then that you may have a "weed", or an unwanted plant. kimmq is kimmsr

The top picture looks like wild onion and the bottom picture looks like Star of Bethlehem to me. Remember there are probably different subspecies of each. Either way its a bulbous plant and recognized as difficult to control. So treat it with an herbicide while its actively growing. In this case, "actively growing" means treat it in late winter and/or early spring. "difficult to control" means treat it with a herbicide then treat it again after two weeks If you do not like herbicides, vinegar is supposed to kill top growth down to the roots. It will grow back so you have to keep treating it with vinegar till it loses all its energy reserves and the root dies. Again, more difficult with a bulbous root plant. Also there are oil based herbicides on the market now that use clover oil and other oils. Though you should read the label to make sure it works on bulbous weeds such as wild onions. Else hand pulling is an option

You could rake the area to pull out the now dead unwanted plant material, loosen the soil a bit and in a couple of weeks toss some grass seed there and spend the next several years fighting to keep the grass growing and the "weds" under control. or you could take the time now to properly prepare that soil. A good reliable soil test, Colorado State University does this for $35.00, is a good start and will let you know what needs to be done to begin getting that soil into a good healthy condition to grow grass. These simple soil tests, 1) Soil test for organic material. 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. For example, 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 your soil will smell, to a point. Too much organic matter can be bad as well. 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. may also help. Your Colorado State University people will also have some good information about growing a good healthy stand of turf grass. kimmq is kimmsr