There is always a risk of adding something unwanted in your tank. Live rock, frags, chaeto from a friend...all can add things to your tank. It is important to check what you add, and if you miss something, which you may very well, then deal with it. I have managed to keep Aiptasia out of my tank but Asterina's made it in on a frag, it only takes one.
I wouldn't worry too much about it, Garf is a decent organization, though at times they remind me more of a cult compound than a business. lol It will arrive and you will put it to good use, I would sump it though and sift through it first.