Looks like red slime algae -- aka cyanobacteria. Which is normal to see in a newly set up tank like yours. It is caused by excess nutrients in the water. What are your water parameters (pH, ammonia, nitrites, nitrates)? How many fish do you have and what kind? How much do you feed, what kind of food and how often? Do you do regular water changes? If so, how often/how much, and do you use tap water or RODI water? Also, do you have enough flow? Do you have powerheads moving the water around in that area?