Heh, I didn't know so many people didn't know what rock anemone is.
Rock anemone comes in many color forms, pink, green, brown, and purple. AnemoneBuff and Cubera are both right. Rock anemone are only harmless depending on what is kept with them. Anemones, whichever kind it is, will not refuse a diseased sicken, and weak fish that wonder into their reach. It is usually the aquarist to be blamed for their anemone capturing their prized fish, for not research ahead.
I've kept one for 3 years and it's a green one. One of the most hardy inhabitant of my tank and has gone through a high temp spike in excess of 92F. In their natural setting, Rock anemone originate from the surf zone and is not uncommon to be exposed under scorching sun for hours at low tide.