HELP!!! what is wrong with my Zoa's

Hi :) My name is Karlotta and I am quite new to reefing. I have a 50 liter Nano Aquarium and all my coral is doing really well except one of my Zoa's... For some reason there is this white stuff on them some of them... I don't know if they are melting, spunge is growing on them or something else.... I'd really appreciate some help!! :)