I have multiple pithos in my tanks. My experience with them has been decent, however, I find them to have a big appetite and are lazy at the same time.
They don’t like to climb it seems and stay close to the sand bed. If I bring the algae to them, they devour it. If I leave it at the tip of the rock work, it stays there untouched for the most part. I noticed they prefer small algae. As an example, I have significant hair algae at the top of one of my reefs. I can pick up a pithos and put it in the algae and there’s no impact because it’s tall and flowy. Do some manual removal to thin it out and then give it to them, and they clean the rock like no other. Not necessarily helpful specifically for your bubble algae but just some insight on my experience with pithos crabs. They demolished my bryopsis though