Definitely sounds like cyano to me and it isn't found just in marine tanks. Try siphoning out as much as possible then totally black out the tank for 3 or 4 days. Do not feed the fish during this time either. At the end of this, do another thorough siphon. If this fails you may have to use Erythromycin - a prescription only medication and used as a last resort.
Better to find the cause. Could be too many or too few phosphates, too much light, or it might have come in on an infected plant.