Moss changes conditions at a pot's surface. Check the main plant's roots and watering needs before deciding whether a moss cover belongs there.
Moss can be decorative
Moss provides a soft surface around some bonsai and other displays. Whether it belongs in a particular container depends on the main plant's needs and the conditions below the surface.
Check the compost underneath
A green moss layer does not prove that the whole root ball has enough water. Check moisture below it and make sure water reaches the compost rather than running across the top. Keep drainage holes open.
Match the plants
A planting that stays damp enough for one moss may be unsuitable for a succulent that needs a pronounced drying period. Do not increase watering merely to preserve the green surface around a plant with different requirements.
If moss appears unexpectedly
Review the pot's drainage, light and watering. Remove part of the surface layer if it prevents inspection, taking care around shallow roots. Moss is a clue to local conditions, not a diagnosis of root rot or a reason to apply a fungicide.
Related reading: Living Pot Covers: Moss, Sedums and Compatibility.