Hmmmm…it prob is green spot algae. Here is some ways to treat it. 1.reduce the lights with a light dimmer. 2. Increase maintenance.3. Get so,e algae eaters such as,snails,shrimp,plecos,depends on tank size. 4. Put lots and lots of plants there. What size tank is this? Java moss can attract algae and it is super hard to get off…