The information in this post can be very useful for beginners. Personally, I have been stung by some of the above items and I can attest to how painful they actually are. However, like stated above, the stings put out vary with different people. Me, myself, they have never affected me too badly. I have been stung by montipora (aka fire coral) three times, and though it burns/itches for eight to ten hours, it does not affect me too much. However, a friend of mine was stung by an anemone and was hospotalized for an entire week because he was allergic to that particular organism's sting.
As for the jellyfish, I believe they can be a little dangerous, but the mushrooms and polyps should be absolutely fine.
As for the jellyfish, I believe they can be a little dangerous, but the mushrooms and polyps should be absolutely fine.