didnt try find out what mushroom it is until now...
Its call Lavender Mushroom, Rhodactis sp.
some information from "the marine centre"
The marine centre
This had been inside my tank for two years.
Its growing bigger and touching my sps.
Lets start from $5
end on 15 Feb (sun) @ 1200hrs
collection at Tampines.
It is the original cabinet. The thin plywood was remove cos It had corroded by water splash from the sump tank.
The main frame still intact, anyway, can inspect the tank before you decide.