Since it is on a rock with xenia and all the xenia I have had doesn't like being in the air long you have a couple of options next time you change your water and siphoning in that area run the tube over it and suck it off hopefully in tact. The other one being when you are changing water take the rock out and place it in the water waste water and take it off there swish it good and put it back in your tank.
Emerald crabs are hit or miss when it comes to bubble algae.