Certified GKH Gardening Expert
Can you include photos? It sounds like a fungal infection, but it would be hard to be completely certain without seeing it.
Watering will be the number one culprit with this issue. Also, this issue will usually resolve itself after cases of cold or hot weather. You can help this along by spraying any areas with a mix of peroxide and water. You should mix this at 1 part peroxide to 3 parts water.
This article will give you more information on growing this grass: https://www.gardeningknowhow.com/ornamental/foliage/muhly-grass/growing-muhly-grass.htm