toadstool leather in trouble

[poo-tang]

i got a leather toastool 2 and a half weeks ago, it a had a brown spot on it but it kind of dissintigrated. i thought all was well until it re appeared and now is spreading rapidly... many of his fingers,nubs? are completely retracted and he is falling over , the stalk is thinner and bent badly. i think i need to do something drastic, cut the crown completely off? parameters are always good in my tank as i test once a week including yesterday.
Nitrate 0
phosphate .1 or less
Ca420
Mg1300
alk3.6
all other inhabitants are doing fine.
it in thebotton of tank low light low flow position