It's black beard algae. I'm really good at growing that, so maybe I should tell you what I do with my tank, and you can do the opposite
I hate BBA. It can totally ruin the look of an expensive Anubias
Things that have lessened my BBA- not using my blue LEDs.
Only using my lights 6 hours a day. This is hard! Not because I don't have timers, but because I want to see my tanks!
I think if I could suspend my lights so there was a greater distance between the light source and the plants, I'd have far less trouble. The plants with leaves that are closest to the light get more BBA, and sooner than those that are further from the light.
Rivercats has posted some great tips on how to deal with BBA- if you search the forum using the term 'BBA' and username 'Rivercats' you'll pull up some of her posts on the topic