Earth Morphling: sounds just like a stronger garboid or antlion, earth has enough such creatures. Perhaps
harden: gain +0/+3
Entropy: sounds more powerful then the other morphlings, it's ability should cost more to activate, probably 2 quanta.
Fire: Its health should not be higher then its attack, I like Jub's idea, 5/1 -> 6/2. Ability cost is too high when taking Deflagration, steal and pulverise abilities, should be 2 fire, especially if its health is reduced.
Gravity: stats are better then a gravity dragon and the cost is the same (or as my suggestion, less), not to mention it comes already with momentum, perhaps if you lowered it's attack power. Suggested-> 5/40, 6/50.
Light: gravity pull is a status effect (such as momentum, adrenaline and poison), not an ability (such as destroy, vamarpic, devour), and will therefore not override the ability of the creature. If the morphling still took the damage it reflected it would not be OP. (aka if you delt 10 damage to it with a dragon, it would die but deal 10 damage to you).
Water: I love it.
All the morphlings (with a couple exceptions as noted above) sound rather over-priced when you consider it takes 10 quanta to get one unupped. I'd suggest costs along the lines of Morph: 5 quanta +2 to activate, upgraded 4 quanta +2 to activate.