On the back says: 'Keep or Rehide' 'WPG Rocks'. Just found there is a fcebook group, artists hide them to be discovered. Not an English speaker so I am not sure if that 'keeps' means that I could bring it with me, so I rehide it to be found by another person :)
Well, in regard to the visuals I would say that a painted rock doesn't harm much (although I'd also say that not every graffiti is bad either)
But your point about the paint being a source of plastic pollution seems more important. Maybe a biodegradable paint like oil (I'm sure not every pigment is biodegradable though) or chalk would do