Hi Scott,
Yes emerald crabs do eat bubble algae but they also eat a lot of other things too so there kind of a 50/50.
If you have isolated patches then I would suggest removing that rock from the tank and removing the stuff. I know this seems harsh but bubble algae can become a real fight when it gets a foothold. If you try to remove it in the water column, what happens is some of the bubbles will burst releasing more algae spores back into your tank-thus why I suggest removing the rock.
You could also try some other things to help keep algae to a minimum like: more water changes to reduce any nutrients, bumping up the alkalinity a little, shortening your light cycle, or after removing the algae you could increase flow in those problem areas.
Just my two cents, as I have alway's been an extremeist when it comes to algae
