Can you tell a bit more about this tank? Water parameters? Hours of light, algae problems, filter exposed to light? Is this color of green elsewhere in the tank (bottom, between glass and substrate, on decoration), does it have a smell?
The water is fine I have petco check it the other day. It's a 5 gal tank with a betta and 3 danios. The light was going 24/7 but after finding out she was doing that I told her to keep it too 8-10 hours of light. The green is only on the filter no where else. I know green stuff is algae but this is a bright color green almost a kind of blue ish. In general it's weird and from having 4 fishtanks myself, I didn't think it was a normal thing