Japanese stilt grass, microstegium, is nasty and invasive. Do your best to
control it.

Here is some info on it:http://www.nps.gov/plants/alien/fact/mivi1.htm

For control, I've found that Ornamec herbicide works well and does not harm
most of my ornamental plants. Manufactured by PBGordon. This article has info
about treating the stilt grass:
http://ipm.ppws.vt.edu/weeds/Microstegium.pdf

Get rid of as much as you can before it goes to seed in late August/September.
The seeds are viable for several years and will spread throughout your yard.

Good luck.