That sounds like mildew from moisture. Try soaking a rag or paper towel in vinegar and then applying it to the stain in a place on the coat that isn't obvious. Let it sit for 20 minutes or so and then spot wash the area.

-- Was your coat cleaned or washed before it was put into the storage bag? If the answer is "no", This could be a regular stain that has become permanent. If the coat is washable, try pretreating the stain and then wash it. You have nothing to lose, at this point.

-- If the coat was cleaned or washed before you stored it in the bag, the stain might be from out-gassing of the plastic, which is made with petrochemicals. Outgassing of plastics can do serious permanent damage to fibers. It has been known to sometimes eat holes in cotton fabric.

Decades ago, I ruined a lovely cream color cashmere coat by storing it with the dry cleaner bag still on it, after I picked it up from the dry cleaners. The spots were the same color you describe and in various places. They never came out. Now I know to never store anything of value in plastic. That was an expensive lesson for me.

Good luck! Let us know what you try and how it turns out.