serpent stars

[mpishi]

i've had two serpent stars (black) in a 75 gallon reef for about three years ago
didn't realize what monsters they grow into
about a month ago i noticed several dozen small serpent stars all over the tank
i've had them as hitch hikers before but i haven't added new rock in about a year
any similar experiences out there?
my gut also tells me they will be cannabalistic given a chance - is that right