BAd algae

[GaryP]

It sounds like it could be cyano.

To get rid of it: increase circulation, siphon off detritus, do water changes, use a phosphate absorber and activated carbon. Small hermits will graze on it. Adding detritivores may also help. Siphon out as much of the cyano as possible when doing water changes.

Gary

[Ram_Puppy]

you can even go after it with a toothbrush while you siphon, and yes, it is bad, it can overgrow corals and kill them. It is more a sign that something is not properly balanced in your tank, as Gary pointed to, Phosphates. What he said is what you need to do.