I have two bettas in a divided 5.5 gallon setup with a filter and some java fern and anubias. Lately, I've been having a lot of algae problem, at least, that's what I think it is. I normally do a 20% water change each week, but not I've upped it to twice a week, each time trying to scrub off some of the black-ish green stuff. It keeps regrowing on the plastic mesh dividers, walls, plants, and gravel... pretty quickly too.
Here is a picture of the stuff I keep scrubbing off.
What is causing this, and is there anything I can do to prevent it from coming back so fast?