I assume you mean its growing on top of the compost within the pot and not actually on the plants themselves? If that's correct, scrape off the top, looser part of soil and moss, back to where it becomes solid with roots. Now topdress with fresh compost. Alternatively, repot properly - moss grows in damp conditions where the soil's not been disturbed for a while, so this might be a signal that your hostas need to be potted on, using fresh compost.

Or after doing the above, top dress with grit.And make sure that the drainage holes in the bottom of the pot aren't blocked.