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It is chickweed. It is considered invasive, but doesn't usually cause too many issues. They are also considered edible! This article will offer more information: https://www.gardeningknowhow.com/plant-problems/weeds/herbal-use-of-chickweed-plants.htm

Chickweed has been enjoyed for generations by many people. (Including myself and family) If you are sensitive to the saponins present or have an allergy to it, I would definitely not consume it. Normally, they are not in enough concentration to harm anyone. Many bean species contain the same saponin groups, as well.

This collegiate study from the Nebraska Extension will give you more information:

https://communityenvironment.unl.edu/chickweed-0

That looks to be Chickweed. This article will help:

https://www.gardeningknowhow.com/plant-problems/weeds/how-to-kill-chickweed-best-way-to-kill-chickweed.htm

Here is an article for more information on the plant:

https://www.gardeningknowhow.com/plant-problems/weeds/herbal-use-of-chickweed-plants.htm

It's either carpetweed or chickweed coming up. Either way you will want to get rid of it. Grass seed often has a percentage of weed seeds included. You can see the percentage of weeds on the container in the ingredient list. Since you are growing new grass, you need to wait before using an herbicide. Wait till you have mowed three or four times.

https://turf.purdue.edu/part-iii-controlling-broadleaf-weeds-in-newly-seeded-areas/