Algae tends to grow on the snail's shell. This is good for them as camouflage.
You lucked out the pink is coralline, which will block the other algae from growing in that spot and help spread it around.
This is why we don't get the pretty snails in the hobby, they always change color and toothbrushes don't get coralline off. Jeweled top snails would be popular even as a coldwater species just for their hermit crab shell potential if it weren't for this.