In general, " DON'T TOUCH " is good underwater advice. Some coral is very fragile and breaks under your fins, some stings badly, like the yellow tipped fire coral.   

On sandy reef platforms, watch out for the long black spines of the sea urchins. These spines are very sharp, brittle  and painful when lodged in an incautious hand or foot. However sea urchins quite passive, and you have to do the " ATTACKING", by stepping on the or attempting to pick up. There are many varieties of sea urchins ; avoid  them all.    

Another watch word is, " BE OBSERVANT". Stone fishes, well named, are quite common on the reefs and are a  sedentary but less conspicuous than the sea urchins, and have very poisonous spins. Shoes should always be worm  when walking on the reefs. Do not pick up or step on the cone shells and jelly fish.    

It's really common sense to respect what you don't know to be harmless, or conversely to respect what may be harmed by your interference. People, not marine animals, are the guest in the undersea world.