I'm not sure I want to try and disturb it to see if it's attached ?It looks like it planted itself? I've read of them shedding tentacles before. If it appears to deteriorate remove it fast but if it's holding onto the rock just give it time and watch closely IMO
The foot near the rock doesn't appear to have the mucus looking stuff on it, but not entirely sure it's attached yet without disturbing it. I'm not noticing any movement with it either other than the tentacle moving but that could just be the weak flow around it.