I had them once and they don't eat corals.
However, in my experience they do like to sit on soft corals like leathers, mushrooms and ricordea. The harm they do is by blocking the light. They are easy to get off the coral, it's not like they are stuck on, But they collect back on pretty quickly. I ended up sucking as many out as I could several times over several days and then getting a wrasse that would eat the leftovers.