Actually all corals will sting, it just depends on whether our skin is thin enough for the nematocysts to penetrate to cause real damage. Most corals won't be able to do much harm to human skin, with the exception of fire corals. Anyway, carpet anemones are quite strong in their sting, you don't feel the pain, but you can feel the 'stickiness'.
Some people have allergic reactions to corals too.