No, they are EVIL!!!
I had a pretty nice polyp rock with all kinds of stuff on it. Over a few weeks it had less and less polyps on it. One day i go in and my dang emerald is looking right at me picking off a yellow polyp and eating it. He had eaten almost the whole little patch of them in just a few days.
Needless to say, he got a trip to the lfs and i will never get another. Without question, they will eat polyps just to make you :mad2: Oh, and my rock had absolutely ZERO algae on it so i know he was eating the yellows on purpose!