Hank,
It looks similar to stuff I have on the underside and dark areas of my tank that grow pretty slow.
If it doesn't smell bad and feels solid, I would leave it alone, but place it away from the light.
It's probably some sort of sponge or colonial animal that was growing on the underside of the rock.
If it's slimy/stringy soft and/or smells bad, it's probably a bacterial/fungal film decomposing
something that has already died. Brushing it and/or siphoning it off will keep the tank cleaner.
Personally, if you are unsure of what it is, I'd just leave it alone. What's the
point of having live rock if one kills everything on it. Whatever it is, it will
self-limit eventually. If it is a colonial animal and you kill it then you definitely
will have a bacterial/fungal/ammonia spike finish it off.
Just my 2 cents...