That goo is from the roof sound deadening panels. They have lost their adheasion to the roof and are falling down and sticking to the glass when the roof is opened. This is a fairly comon issue with Ml's. The glass panel is removed to repair. The old stick-on panels get scrapped off ( usually with a little heat) and replaced with new magnetic / sticky design. MB issued a DTB about this
Some time back limiting the flat rate time to 3.5hrs for 1 panel,
or 4.0 for both. Of course you do both. Back in the day, we used
charge MB warranty around 15hrs for headliner R&R. Yahoo!
The dealer should stock the new magnetic verion panels.
DR.D