I don't think that there are parasites that travel on mushrooms really... Either way I'm sure everyone would just suggest QTing the rock in a tank for 3-6 weeks just to be on the safe side. I would imagine that the worst you could contract from a mushroom rock would be.... flatworms (probably not the Acro Eating variety), aiptasia, cysts of ich that spread to your tank, Zoo or monti eating nudibranches? I dunno. QT is the best policy.