u/Efficient_Divide2020

I’ve been scanning hundreds of slides my grandfather left me when he passed away. One set of slides has these leafy looking spots all over them. I collected all the slides and bagged them together, confirmed they are all form a single reel. They’ve been stored together for 15+ years so I don’t think it will spread anymore?
First, does anybody know how it happened? Did he shoot a roll with a dirty lens?
Second, does anybody have a recommendation on removing them in photoshop or Lightroom?

Thanks for any insight.