This is a cyano, or red slime outbreak. It's very common in new tanks. Keep fighting it, but above all don't panic.
I do not recommend every using any of the red slime remover products, because unless you fix the root cause, you'll do a lot more harm than good.
Usually a new tank will go through several blooms of unwanted algae over the first 6 months or so. Maintain your water quality and they will pass even though the tank looks terrible for awhile.