I believe “Mote of Light” and “Little Light” are just a colloquial way of describing Guardians and Ghosts, either with condescension or affection. Not that the Exo Stranger was being affectionate; for Ghosts, that could be a literal description.
A friend and I talked this sort of thing over on FanFiction.net, where she was bouncing ideas off of me of how plausible her ideas were for stories (in addition to taking apart Mary Sues, but that’s another story), and we got into a discussion of how Ghosts actually function as our “horcrux”.
Basically, what it came down to was this: Ghosts were either a “storage tank” of Light that just needs to be jumpstarted with external Light – similar to how Guardians can self-revive in Patrol areas or need Guardians in Darkness zones – or else they absorbed Light passively and used that to keep themselves alive but lose some through simple emission. This latter information was determined through various Grimoire and flavor texts, like that one Ghost hiding among the Cabal, saying it was so far from the Traveler and fading.