Unknown critter

[Hangfire]

Thanks everyone! They are sessile, and do appear to be shiny. It's located in a more shelter part of the tank, with good but moderate flow. I've been feeding the tank every three or four days with some filter-feeder food - all the tube worms and the sponges seem to be doing very well as a result. Can't wait to see how this one changes as it grows. Thanks again for the responses.