It happened to my dragon stone and the fish were fine... Just a character of the rock I suppose... I wouldn't be too concerned about the spots BUT I would keep a close eye on it to be sure it doesn't keep spreading and kill the plant.
It is an Anubias congensis, and I have not tried wiping the stuff off of the rock. I did try a lead weight for one of the plants, not that one but I couldn’t get it to work so I took it out. Here is what the rock looked like before. I know it is not a good pic of just the rock, but I’m not home and don’t have one. It shows the affected spot on the rock