[jlm86]

My 55G, about 2 months old now, sand bottom.

BRIGHT green patch on sand about 4" in diameter....really weird looking....

any thoughts?

 

[Fevz]

It's a ciano bacteria

 

[prov356]

Vacuum it out and stir and vacuum the sand. I've seen it get started in mostly newer setups with a bit of organics in or on the substrate. Increasing water flow over that area seems to discourage it. Cyanobacteria isn't harmful to the fish. Google it and you'll find a whole lot of info.