I'm not sure how you would scrub off Digitates. But if it is only on one rock then pull the rock, if you can, and let it dry out for a while. Are they like single little strings floating in the current after lights out, or are they "feather duster" in appearance. I believe that Digitates are the single little strings and colonial are the feather duster type.
Either way I would pull the rock if possible, but I have read that Digitates will disappear as quickly as they appear. (I have them and have done some reading)