125 g keeps gettind mold on lid????

[Daniel475]

hey guys on my 125 gallon it keep getting mold, the style of lid thats on it is more like a cupboard lid.. it has 3 slots at the back for air flow but it doesn't seem to be enough?? and i dont really wanna leave the doors open just incase one of them jump out , ive been treating with melaxfix latley for cloud eye could this be doing it??
thanks alot
dan