cyanobacteria is kinda like a red slime algae. On a side note I would lay the feather duster flat on the rocks and not keep it in the sand, bacteria can really hurt it more while in this state.
Mike
I have 4 feathers and only one has every thrown it's crown due to stress of a bi color blennie. It never was hanging out of it's tube though. Try gently pulling the him out ( careful with the anchor foot) and put him between two rocks where he has some cover. If you have a hosp tank it may even be better due to the bacteria and nothing will bother it. I usually use doses of Melafix either way. The crown will grow back with 6 to 8 weeks.
Just my opinion
KC in KY