Well, my first inclination was jointing compound but I'm not convinced that quantity would be in the cylinder. My second thought is foam insulation used during the tank manufacture which has somehow got inside the cylinder.
Maybe the most likely explanation is the compound used to help release the tank from the mould when it was pressed which wasn't cleaned off.
I would follow Proptec's advice and contact the cylinder manufacturer and ask their opinion.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!