Daylithos sp. Flabelligerid worm - A.K.A. Coral and Rock-boring Cage Worm. The fan is the cephalic cage that the common name comes from (it's a bunch of specialized chaetae - bristles - that spread out around the head) and the tentacles are the palps (the feeding tentacles).
Neat, and this is the first I've personally seen an aquarium, though they're not unheard of - they don't seem to cause issues (though I imagine you wouldn't want too many for your corals' sake, since they dig through the skeleton for their tunnels).
I wouldn't be able to ID the specimen in the tube, but if it dies or you otherwise remove it from the tube, I could walk you through getting pics to ID it at the species level.
->It's worth noting here that there is almost no information available on these worms' diets - it is suspected that most Flabelligerids feed primarily on "phytodetritus" (be that phytoplankton - live or dead - or detritus from various algae species, I'm not sure) and may occasionally supplement that with meaty detritus or (thought to be more likely) things like bacteria.* If you are able to note anything about its diet, that would be great information, and - as far as I know - useful info to the scientific community that researches these critters; please document any info here for me!
For some pics to compare with (both of worms in the tubes and out):
*Source (the link in the quote below now has the paper behind a paywall - I hate when sites lock papers away - but the link to download the Appendix still works for free; the Academia.edu link is accessible with a free account, and you don't need any special credentials or anything for the account):
Diet of Worms Emended: An Update of Polychaete Feeding Guilds
www.academia.edu
www.annualreviews.org
