Could be the results of past condensation in a dirty lens, or even remains of the helicoid grease and ground down metal from the same. You'll know more once you get to it - which you can't avoid short of dumping the entire lens...
This should be totally cleanable. That looks like a Minolta Rokkor 58mm. Remove rear mount, unscrew the rear cell. This sort of spots is almost always between the cells. You should be able to carefully clean it and make it look like new again.
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