Yes, some of the Miscanthus cultivars do occasionally revert OR sometimes they seed into the established clump and after a couple of years you are presented with a surprise. If left, the solid green part will be far more vigorous than the variegated bit and will eventually take over.
You can;
1. Dig out the green bit now if it is really bothering you however it is a bit late in the growing season for that to be a great idea.
2. Cut the green bit back now so you can enjoy the variegation and deal with the problem in the spring.
3. Leave everything for now and in the fall, cut back the green bit so that next year you can identify which are the good bits and which are the variegated bits and cut out/divide the grass in the spring.

Hmmmm....
just noticed that you didn't ask what to do ~ too much information??