Black beard algae. Whether it is bad or not depends on your outlook. It's rough if you have a planted tank but if not, it can be okay. Since you have Africans, I might leave it.
Sometimes it is easier to ignore nature than fight it but then that is a personal decision for you. I had a 75 full of it for a time but at some point the tank changed and it went away. I currently have it lining the tops of rocks in a 125 and people think it is great but then those are not planted tank folks who know algea is bad. Go figure! Many things are just a matter of what you like in YOUR tank! Sometimes I fight it, sometimes I leave it.