Help what's on my ray

[Train]

So both my motoro stingrays have developed this on there belly and I have no idea what it is

 

[Train]

 

[Woefulrelic]

Do you have an exposed heater?

 

[MEDHBSI]

dido to the heater.
is there any sharp rocks in the tank?
whats your substrate?
how long have you had the rays?