If it's hairy, black algae (often called black brush algae), C.
siamensis (Siamese Algae Eeaters) will eat it. It is virtually impossible
to remove from plants by hand. Cut off the worst looking leaves,
introduce the fish, and wait 4-6 weeks for most of the algae to disappear.
First, you'll see that new growth is algae free, and then you'll see the
older algae eaten to sort of a blackish residue - you may want to remove
those leaves once the plants have plenty of algae free leaves.

http://www.thekrib.com/Fish/Algae-Eaters/

It`s a good dissolution too