I would not keep an adult SAE in a 20, but a couple of juveniles might be OK, if they deal with this sort of algae.
I do not think it is hair algae. I had an outbreak of this in one tank, and it did not look like any of the algae commonly called hair algae. It is more of a slimy mass without a lot of form, and debris gets stuck in it.
I think ua hua is right.