Little white flowers? I don't know its' name either, but looks like a weed I had, perennial. Took a few years of pulling/mulching when it sprouted to get rid of it.

I don't spray chems on weeds, but if you do, read the instructions well. Generally, you would want it to be a warm, sunny, NOT WINDY day, after the dew has dried. RU is absorbed by foliage, so no need to use any more than necessary to coat foliage, or put it anywhere else. If you pull/cut something at ground level, then spray when it's begun to grow back, having used stored resources to do so, you'll get optimal devastation. For example, weed-wack or cut to ground several times while waiting for the correct weather. In that weakened state, a little poison will be much more effective when you use it at the proper time.

I have something similar which I think is diodia virginia / Virginia buttonweed. 4-petalled flowers.

Mine grows in grass and is taking over. It can't be pulled up, and needs to be dug.