I know what you are talking about as I am sure many here do. These are very common on live rock and although they do not get very big, they add to the diversity of the tank. Pretty cool, aren't they. I have notice at least 6 different spieces in my own tank. Some are translucent while others are white, pink, red, and blue.
I love them. They need no special care and take nutrients that they need directly from the water.