lime green is roughstalk bluegrass until proven otherwise. Unfortunately, yours is seeding already. I would definitely hit that area with roundup using a glove--especially if you let it grow taller than desirable grass, as in your picture. Pre-emergent in fall is useful for areas that have heavy infiltration. I'm afraid mulch mowing and core aeration can spread stolons everywhere. I find spring roundup the best way of control when it seems to be growing faster than anything else, so is easy to identify and kill.

It seems like this is something that Tenacity should be able to take out as well. Anybody know?

Tenacity will light it up but probably will not kill it. Certainly not at label rate.

I have some of it, too, and am going to be doing a broadcast treatment of tenacity in the next week or so. I'll guess we'll find out what happens! Thanks.

So what happened, Daniel?