First off, a question. If a creature has (max) 1 hp and dies do you get 0 healing (as the card seems to indicate)?
I think the concept is great thematically. In real life, compost is of course used as fertilizer for plants so that they may grow stronger. In that same way, the dead creatures are used to fertilize the Elemental(player).
As far as the the mechanics goes, it seems like you have already done a good job balancing them. One half of the max hp of a creature is pretty reasonable. I don't foresee this card being abusable.
If Monolife were to succeed, it would be in killing the opponent's creature using a Thorn Carapace. I think this would really give life a boost. Empathetic Bonds are not that useful in Monolife and Compost could give Thorn Shield a much needed buff in my opinion.
Duos or Rainbows could work quite well too.
Armagio immediately comes to mind. Mid range cost and high hp means up to 15 healing. That wouldn't override heal, which is 20 on command, but it is a nice decent payoff and easy to set up.
Schrodinger's Cat would work as a very expensive shard of gratitude. My understanding is that unupped, you would get no healing from it. Upped (stats 1|3) you would get only 1 hp for 2
, which is really expensive. Nonetheless, in combination with other cards, there could be some synergy.
Immolation in a Life/Fire Duo would work, but not well. Fortunately, it is reasonable balanced as both life and fire have weak HP creatures so they would not benefit from a strict duo that much. 1 hp creatures (as I understand it) would not give you any healing so immolation would have to be used in a trio to be very effective.
Perhaps growing a lava golem in a Life/Fire/Earth trio and then immolating it late in the game for an EM. Or otherwise using a Life/Fire/Water trio and forest spirits in a similar vein as the lava golem strategy.
I think this card is a very nice idea and would fit into life without taking away from Life's current cards. Looking forward to better art. Dead fish do not exactly convey your idea that well.