Post split off from this topic by Moderator
If your CYA isn't too high, hit it with a few pounds of 60%+ granular trichlor, otherwise liquid chlorine. Though I used to absolutely gear away from all copper algaecides (except for last resorts) and will probably get **** from everyone for recommending it...well, it'll probably knock it right out! I'd follow up the super chlorination with a copper or silver-based algaecide, run the pump overnight, and re-chlorinate the heck out of it the next day, because the FC will get eaten up. You might want to follow up with a monopersulfate afterwards to clear up any remaining cloudiness from dead algae and CC. Once everything is trapped in the filter, clean it! Clean it good. This process is what I do in severe mustard algae cases, and works EVERY time. Don't get into a habit of relying on the stuff, though.