Where do you live?

What kind of grass do you have?

Pennsylvania. Cold season grasses. Tried to establish a lawn last yeat with a contractors mix of kentucky blue grass and fescues. we have a new build and this is growing with the grass. the roots are very hard

Thanks. That helps to narrow down the weeds and treatments. My guess would have been very wrong.

OP What are you asking about. The grass in the background or the broad leaf light green plant in the first picture.

Light green plant

It's not the knotweed :-) That is also known by the common name of prostrate knotweed and it is a flat, ground hugging weed, even in the seedling stage.

While I agree that identifying a weed like this provides some satisfaction of "knowing your enemy", it is really not necessary for treatment. As noted, a broadleaf lawn weed herbicide should work to eradicate.