It's called "potting" compound, it's basically an epoxy resin. If my memory is correct, the back-side of the PCB assembly is not constrained by potting. If you need to remove/replace an LED, you will have to physically remove the board assembly from the housing face, and then CAREFULLY chip away at the potting to expose the component that needs to be de-soldered. Once the LED is replaced you can replace the potting with any epoxy resin material - however small-batch potting compound is available on Amazon.