ISpeakForTheSeas said:
Normal limpets are totally fine, but there's some speculation about Keyhole Limpets possibly eating SPS corals. If yours is a keyhole limpet (which it looks like it may be, but the photo is too grainy for me to tell you for certain), I don't know that anyone has actually confirmed that they eat SPS (much like how people believe "Asterinas" - actually Aquilonstra - stars might eat their corals: it might be a species specific thing, it might be a you have way too many starfish so they're out of other food options thing, it might be they're eating the slime coat/mucus on the coral rather than the coral itself, or they might just opportunistically eat unhealthy corals).
If it has a little "keyhole" or bullet hole looking hole on the back of it's shell (where the shell comes to a point) then it's a
keyhole limpet.
Personally, I'd assume keyhole limpets are probably safe, but I know a lot of people would rather play it completely safe and not take the risk.