Let me try and clarify some information for you. Your level 20 guardian is able to earn motes of light in lieu of gaining a level through experience. All experience you earn go towards that mote of light progress. (If you check your character in game on the menu screen, hover over the 20 to see the progress bar). Leveling up with your other characters doesn't award motes of light. You can obtain motes of lights in various ways. Most common way is through the experience bar on all characters over level 20, or through rare engrams. Its worth noting that others have said that you can earn 3 motes of light through an eris morn bounty each week, however it requires running the nightfall, which you cannot run as a level 20. Edit: In case I confused you, you can still attain levels after 20. You'll need Gear with light. This light will allow you to ascend past level 20.