Do a search for red slime. There are several ways to remove it. First is suction all of it out and increase your flow to that area. Second is determine what is causing it, overfeeding, too long of a light cycle, not the correct spectrum of light, or is it just a new tank? Third and IMO don't do this but you could use red slime remover.
The reason I say don't use red slime remover because is made a mess of my tank. Alot of people around here will tell you it works great, and yes it did work it just made a mess of everything.
How much flow do you have in your tank right now? I would say you want a bare mininum of 15 times your volume of your tank per hour. So if you have a 100 gallon tank then you want 1500gph turnover. Now this don't have to all be run through your filtration system, you can just add power heads to achieve this. And I don't think hang on skimmers really count because they don't move too much water at all no mater what the gph on the pumps are.
You also don't want to have any stagnet flow areas in your tank either.