help with my grey mould problem

hi im new to this an my first grow of 3 plants is goin well untill i saw some grey mould on my biggest plant? it only attacks the stems were they join the main stem,i dunno what to do/how to cure it? any help will b greatly appreciated

pics would help

I heard that the cause of this is not removing the dead fall from the plant . anybody else hear the same thing ? also ....what is the best way to stop this ?????