They are like velcro. Great cleanup crew tho. Can really get to some stubborn algae that even turbo snails won't touch. Alas, Always a catch. Keep him based on your needs. If you don't have a bad algae problem, i would consider selling if you have valuable corals that arent drilled into your rock or encrusted already
I love urchins, pin cushion ones, since they stay small, doesn't sting you, are immune to hermit crabs and other snail predators, don't die like snails do... so many pros, so few cons. Yes they move stuff around, like any other animal who can grab a shell or loose rock, Not really an issue if you glue it down. If you glue something to a smooth surface, it could pop off, but if you glue onto a rough surface, or something with a feature, the glue/epoxy will enter the feature and anchor it down.