What's on my Pleco?

[Aubertson]

Hi I've only had him a week and he has this white patch on his back. The first day he sat behind the heater the entire day so I thought maybe it's a burn?

 

[goldfishallday]

what temperature is the heater on? It shouldn't be able to burn him, if it is doing this then the tank is unsafe.
Does he have any other hiding places in the tank?
Would you be able to post an image or describe the white patch in more detail?