Hi
It's always a little hard to tell from a photograph, but that looks like red slime algae which is actually a type of photosynthetic bacteria (Cyanobacteria)
It looks great while it's contained, but it is indicative of other tank problems such as low flow, declining water quality, etc.
It will eventually take over your tank and drop your redox potential until nothing will do well.
Using red slime remover (which, I believe, contains E-Mycin) will help, but it is toxic to your nitrifying bacteria.
IMHO you are better off cleaning it off manually and looking for the cause.
If not---enjoy
[hr]
for a while!!