After successfully repairing my 135mm lens, got adventurist and pulled out a cheap ($50 back in the day) no name 400mm f6.3 full manual lens. Hadn't used for years due to clouding of the lens element, but thanks to the "T" adapter design, easily converted from Pentax "universal" screw mount of my original Mamyia/Sekor, then Nikon F and ultimately Canon FD mounts. Had to fully disassemble because the obstruction was on the internal side of the element. Even when fully out, wouldn't clean off. Believe the film was originally mold that tried. Ultimately, had to pull out a Dremel with a glass polishing pad. Since the lens was toast anyway, wasn't that big a risk.
A test shot after the cleaning - handheld @1/250 sec as screw is missing to tighten the tripod collar.