Thanks for the feedback. I understood the concern about a high hit point creature with photosynthesis being too powerful. I thought the -1/-1 would work by reducing the full hit points (not just damaging the creature), but I did not know how to make that clear or if it was possible. I did not want to simply restrict the card to low HP creatures, because one of the motivations for this card was to enable something a bit more robust than a leaf dragon which are easy to take out before they even get to be used.
What do you think of infecting the creature each time it uses the ability? This means you can keep a high HP creature with photosynthesis in the wings, convert as much light quanta to life as you want for at least one turn, but, in most cases, the creature won't last too long. As willing3 pointed out, this card+light dragon + mitosis emerald dragon is too powerful. With the infection modification, you could only generate enough life quanta for two mitoses before the light dragon was dead. And the infection keeps the card's theme of a lichen-like disease.