+1 Cyano. New tank? If so, several solutions out there. Nip it now. Old tank.... why, you need to find out that answer. Fairly common in new tank to go through some diff blooms, so this would not be totally unexpected, but get rid of as part of the new tank set up if new.
If established tank, again, nuke it, but figure out why you all the sudden got.
In any case, treat it, as it is problem-some.