I had a similar situation with my Valencias. I called Bill at GPA who told me this is a common problem. He advised me to use acetone and a small brush to remove the goop. I used a soldering acid brush. He also advised me to rotate the 416's 180 degrees from time to time to keep this from happening again.

The material did not come off easily but with persistance it did come off. I also used the acitone to clean and refresh the surrounds all the way around the cone. Now they look brand new.

The acetone can be bought at Lowes as well as the brushes. It was not such a big deal to remove the goop and also refresh my 416's.
Marvin