For where they are, I would hit them hard woth boiling RODI water. Boil the water, take a baster/pipette and let them have it. Don't stop until they are completely dead and falling off the rock. Then suck them up and toss them. They can grow back from a small bot of living tissue, so best to kill them completely.
Are they localized to those two spots? Check everywhere and get every one you see. Then keep an eye out and get every additional one that pops up, however small. They can, and will, take over if you give them the opportunity.