White dot on emerald crab?

[Gabbone]

Hi guys,

My emerald crab has this white dot on his shell since the first day I took him.
Since he is growing, it seems also the white dot is growing.

I am attaching a pic.

Could you check it, please? Could be a spirobid? Thank you a lot!

 

[Cthulukelele]

That looks line a spinoid worm. Harmless filter feeder. Usually die out in mature tanks but can have a population boom sometimes as tank matures