Underside of Monticap turning brown

[Oba97]

Can anyone tell me what is going on with my monti. Is it normal for the underside to turn green/brown? My second pic is the full tank for reference. I wonder if the top section is blocking the light?

thanks in advance

 

[Drummingbaker2010]

Could it be a slow buildup of algae growing on it? are the parameters in the tank ok with the trace elements and iodine?