mounting a dustcap

I got a recone done on one of my SX, well it arrived and the dustcap is pretty much not attached on the sub only held one by about a 2" piece of the cone. What is that black stuff that is holding the cap on? is that regular silicon? or should I use something else to mount the dustcap? It appears that the black stuff is all the way around the cone where the dustcap should be connected to it it just appears as if it didnt stick.