Yeah - they do release some chemicals that can be toxic. One of nano-reef.com's admins cuts them for propagation fairly often. He usually leaves them in a container outside the tank for a few hours, then puts them back (no transfer of the water, of course).
My best way of explaining the stickiness - I was moving things in the tank a few days ago, and one grabbed my finger. When I moved it, the entire rock the mini carpet was attached to came along - probably a pound or so.
I'd love to have a haddoni or any of the larger carpets, but I'm stuck in nano mode because of apartment restrictions.