That looks like cyanobacteria (red slime algae). You can suck as much out of the water by hand using a turkey baster. Also, it may help to increase flow in the areas where it's showing up, so move a powerhead around. Other things that fuel cyano are the same as other algaes -- overfeeding, tap water, overstocking. You can do a blackout for a few days too, that will help to get rid of it. But again, you still have to remove as much of it by hand.