Hi Matt,
If you want to get rid of it while it's still on the rock, you can try the liquid carbo thing, works every time. Is the rock new? It's a bit funny to start off in just one spot.
Anyway, here's an easy, quick algae killer: Turn your flow/filter/powerhead off for just a minute and syringe a small bit of Flourish excel type product directly on to the algae. It will turn orange in a few days, vacuum it off. Poof gone.
Black beard algae shows up on the edges of leaves frequently but if it's only on the rock, you could simply scrub the rock off and give it a baking soda wash (and rinse!)to change the ph of the surface so it won't come back.
Have you got a better or bigger picture? The nicer algae that looks similar feels soft - the nasty one feels like beard hairs of course.

Racing, shoes and fish. Nothing else matters. Oh, and bacon.