Nail polish will last up to 12 months, depending on the quality and how many times you take it on a plane (the cap always unscrews on me during flights, leaving my purse a -- fume-y -- mess!). See 5 really tacky nail trends.
All fungi are heterotrophic, which means that they get the energy they need to live from other organisms. Like animals, fungi extract the energy stored in the bonds of organic compounds such as sugar and protein from living or dead organisms.
You are correct in your observation that it is a mold growing on the trunks of your trees. But even more correct is that it is a lichen. Lichens are a combination of two distinct plants that have taken up living together to get benefits from each other. The first part of the symbiotic relationship is the blue-green algae that pair up with a fungus (mold) that can't produce its own food stuffs. The blue-green algae produce sugars and starches and amino acids that the fungus uses to grow, and the fungus provides a substrate for the algae to attach to and grow and prosper.