algae or bacteria?

[justinl]

Looks like cyanobacteria. I don't advise chemical treatments unless it's actually threatning something in your tank... cyano poses no real threat; it's just aesthetically unpleasant. More water changes, use RO/DI, think about how much you feed, etc... think about what might be adding unnecessary nutrients into your system. You can siphon it out too if you're not worried about losing a bit of sand. Also, how old is the tank? New tanks go through different stages and cyano is often one of them... if that's the case, they usually run themselves out and the cyano goes away in time.