It looks like blue foam insulation panels.
That said, the chemicals that you use to seal the foam to make a smooth master for your mold are rather important as well. There are a number of ways to attack it. If you plan on sealing the foam with resin, you will probably need to use an epoxy resin which will not melt the foam. Most polystyrene foam will melt if polyester resin is applied directly to them. However, if you were to seal the foam with plaster first, then you could use polyester over it. There are many way to do this and I would suggest some research into the methods related to what you want to create.
As for CNCing the foam. That is the expensive way of doing things, but do not let that stop you if you like. Because the foam sands so easily, many will make a template of a station, glue it to the foam, cut and sand the foam to shape. Messy, but quick and inexpensive.