Only light can damage it, everything else provides healing?
That's basically making it invisible but better, and indirectly buffing Holy Flash if you give it positive health, so it might be OP.
With Negative Health, this effect would make it UP.
Don't card's with zero die instantly?
I would assume the game has no coding for negative, so your effect would work, but protecting it from death at 0 health would probably be very inconvenient to code.
Not that coding difficulties should force a change in effect...
^This. Negative Health is an awesome concept, and I don't want it to die midway.
I think this should be resolved by giving this card the Passive Ability "Necrophage"
"Necrophage" : HP changes from Eclipse and Holy Flash are reversed.
That way Eclipse still buffs it and Holy Flash heals it.