Could be a mini nem that hitch hiked, that mouth says its not a shroom. They have a powerful sting to corals and some people, but usually stay put and multiply very slowely.
My guess is a Maxi-Mini.
Although I need a better picture.
If its a maxi mini its not harmful at all and should color up very nicely.
Also maxis are known to hitchike on live rock
Well I talked to the guys Son who 's dad sold me the rock with the maxi on it, he instructed me it is a Blue leather? Dont know whether to belive it or not? his dad told me its a maxi-min / blue