From my (very limited) experience it targets whoever is injured automatically with a heal spell/oGCD of relevant potency. Enemies targeted will get hit with dots and either single target damage or AoE depending on number. Worked pretty well on Astro, as it also automatically uses cards on the appropriate classes. Does not work well on Sage, as Miqo doesn’t seem to know how to use Eukrasia yet.
When in doubt, run a Trust or Squadron dungeon as healer first and see how well you do.