Sally Lightfoots are "reef safe". However, any crab is a critter to be viewed with suspicion in a reef tank. It's uncommon for SL's to bother corals, but some of them will. I'd watch it closely, but as long as there is no damage to the corals, I wouldn't worry too much about it.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.