Blotchy fillum
I had a similar problem with colour transparencies from as far back as the late '60s. I took them to a "man who knew" who diagnosed it as a form of fungus which grows like those piccies you see of bacteria growing in a Petri dish (if you note they are almost all circular). Just be very careful using any form of cleaner. If your slide is of particular sentimental value why not just accept it as it is and, maybe, try to use some of the tweaks and tecniques in Photoshop or something of that ilk to remove them. That way you still retain the original.
The Ancient Mariner