It looks like a filamentous brown alga. Do you use RO/DI or another type of purified water for your water changes? What kind of herbivores do you have now? I don't know if a sea hare would like this stuff but I'll bet a Diadema urchin, foxface / rabbitfish, or naso tang would. Do you have a refugium? Growing chaetomorpha in a brighly illuminated 'fuge may help outcompete this alga for nutrients.
HTH,
Kevin