It's rock solid, not like the cables glue.
What is it ?
What is it for ?
How do I remove it ?
How do I put it back ?
This is known as "Potting."
Potting (embedment) is the covering of an electronic or electrical device to protect it from the surrounding environment. Most of the time it is for protection from water or moisture and /or to electrically insulate it so that it will operate as designed. This can be done by several methods: potting, casting and encapsulation. Many names have been used interchangeably so there is some confusion of the terms. The most commonly used term is encapsulation or potting.
I have removed it in the past from a PFC module by scraping it slowly out with a screwdriver. This is normally only done to figure out what is in the module, or to determine a failing component. I have never attempted to reverse the procedure, but I assume it would be possible if damage is not done to the components during removal.
Potting Solutions also discusses the removal of the potting:
There are two ways to remove potting compound either physically or chemically remove it. To physically remove the potting compound it is common to heat the part until the potting compound becomes soft and it can be torn off. If it is a high Tg potting compound it could be cooled and then hit to shatter the resin since it is brittle at low (-40C) temperatures. Chemical removal of the potting compound is done by soaking in solvents to soften the resin so it can be torn off. The solvents are toxic so care must be used.