It looks like cyanobacteria. If it has an oily and smooth look and texture, chances are it is. I would try and reduce the nutrients in your system if you can, and it may also be worth increasing the flow in that area. Increasing flow has helped some curb the growth of cyanobacteria for some people. You can also try siphoning it out with airline tubing (but this is only a temporary fix that must be performed regularly unless you solve the problem).
FWIW, products such as chemi-clean and red slime remover have been known to help some more than others. Do some research on this - though chemical control should be a last resort of sorts.