Every year long blades of what looks like grass starts growing in my creeping phlox. I have to keep pulling it out by hand and some of it actually has roots.
Why Does Grass Keep Growing in My Creeping Phlox?
The grass is just doing what grass does. You could of course try spraying an herbicide that kills only grass, as this article talks about: https://www.gardeningknowhow.com/plant-problems/weeds/using-herbicide-in-gardens.htm
However, my experience is that sometimes things don't work the way they're supposed to, and your phlox may end up getting damaged. Pulling is probably the best way to go, making sure you get as many roots as possible. You might try using an edging barrier to keep the roots out once you get them pulled up: https://www.gardeningknowhow.com/lawn-care/lgen/tips-for-choosing-lawn-edging.htm