Looks like purslane, and it's not a clover. You can use Weed B Gon when you have mowed the new grass three times. It is edible if you want to use some in salads. It will outcompete your new grass so you will want to get rid of it.

Looks like henbit to me. Does it get small purplish flowers? I get some of this in my yard also and just hand-pull, it does have small roots and comes up easily.

Mouse-eared chickweed would be my guess if flowers are white. Purslane has glossier leaves.

I think mouse-eared chickweed is a better guess than purslane. They both form mats, but Kaillean is right that purslane's leaves are glossier.