What is White Cottony Stuff on a Fish?

[goldfishbetalover]

First topic message reminder :

My friend has 1 guppy
she was in a tank with another guppy that has sadly expired...
the remaining guppy is now sitting idle on the top left hand side of the tank with a white cottony substance that appears to be growing on his nose...does a fish have a nose? she says that there is also some of this white cottony substance inside the tank itself......
what is this?

[goldfishbetalover]

thank you I will tell her she should do a thorough cleaning.........of everything!!!!