Richard can confirm this, but I believe there are 4 common hitchhiker crabs. Porcelain, Emerald, Stone and Gorilla. The latter two are primarily the ones that need to be observed for potential issues. Some people report no issues, while others report problems and therefore need to be removed from the system.
Ultimately, there is no guarantee with most marine creatures. For example, Yellow tangs in my experience are borderline reef safe. In fact they really should be characterized as with caution as they have a tendency to pick at clams, soft corals and LPS. Not to the point of trying to eat them, but they pick. However, I did have an aggressive juvenile yellow tang destroy a Maxima clam on me. That fish went straight back to the LFS, cursing it the whole ride there.
Ultimately, there is no guarantee with most marine creatures. For example, Yellow tangs in my experience are borderline reef safe. In fact they really should be characterized as with caution as they have a tendency to pick at clams, soft corals and LPS. Not to the point of trying to eat them, but they pick. However, I did have an aggressive juvenile yellow tang destroy a Maxima clam on me. That fish went straight back to the LFS, cursing it the whole ride there.