Well, I'm not sure what in this game is technically considered "passive" but I had figured that this would apply to any ability that had an activation cost. So things like the Hourglass, Lava Golem, Rustler, and even Virus would trigger the healing. Ray of light, would not.
Passive abilities include:
-Quanta Generators (Bioluminscence [
] from Fireflies and Rays of Light, Incandescence [
] for Elite Fireflies/Brimstone Eaters, [
] from Gnome Gemfinders, [
] from Damselflies)
-Devourer (Self-explanatory
-generator/stealer)
-Vampire (Also self-explanatory
)
-Infest (Malignant Cells)
-Pharaoh "evolution" (Mummy)
-Poisonous (New update with Flying Arsenic, Physalia, and Puffer Fish)
...and a few others. This would be amazing on decks with high ability usage (Rainbows, FFQ decks/Nymphs/Flying weapons/etc.),
but the sad thing is that the only card that would benefit from this would be the Archangel, seeing it's the only one with an active skill...EDIT: Forgot about the Pegasus too. So two light creatures can benefit from this card - not bad.