I was actually getting ready to post the very same question about mine (Miscanthus Morning Light). Is your weed/new growth standing up or floppy? Mine are in the front of the clump and not very erect--somewhat flopping on the ground--and also not variegated (i.e., green with white) like the rest of the grass.

It's possible they are volunteers from your OG's, although in your zone, it's less likely than in warmer climates. In cooler zones, although the plant flowers, it frequently doesn't have enough warm weather to ripen seed to viability.

'Morning Light' isn't likely to come true from seed.

I pulled them up and planted them in the back yard. It will be interesting to see what I get. If it's just weeds they will go in the compost. I figure I have nothing to lose. If they are just weeds I can have a good laugh.

Rose

I posted last year that I had volunteers coming up, and quite a few...the three grasses I have are Gracillimus, 'Adagio', and Zebra Grass(altho it was labeled Porcupine grass)....this is the second year growth for the volunteer seedlings....

I'm not quite sure what to make of them...about half of them are 4.5-5'tall,very stiff green blade about twice the width of a Gracillimus blade, white vein down the center, very erect(little flaring out at the top) and starting to bloom along with the 'Adagio'...the others have very fine blades (finer than Adagio) and are the height of Adagio, about 2.5', and no indication of bloom onset...so I'm enjoying them but a bit confused as to what their genetic make-up might be...lol...the Adagio have always bloomed this time in July for me, the Gracillimus and Zebra grases don't start blooming until late September..