Sound like harmless sponges.Hydroids, come in all shapes.It took me a while to find the little white/clear pinapple shaped ones with its halo of spines.
Research many sites, as this looked nothing like most I was seeing online, then I found it after long search....Thanks to a knowledgible person on this site.......
This could be what your seeing,it's a sign of a well managed tank my friend.
ScottbytheSea