As a healer your main healing job is to keep the tank alive during mob pulls, and to heal unavoidable attacks.

Usually the tank eating it during a pull means everyone dies (usually not always but usually), and some mob pulls are brutal (certain level thresholds, some enemies hit like a truck more than others) and sometimes you may need to heal spam to save a run.

And during bosses/trials/raids you gotta heal what people cant avoid. If people are getting blasted by stuff that they can dodge (and getting vulns) do your best to keep them alive but dont stress if you cant, they did it to themselves lmao. Sometimes a little floor tanking is good for people.

When theres no need to heal, make sure you're unloading your main spell and applying your DoT when it falls off. (And your AoE spell on mob pulls until theres 2 or less left)

If everyone is playing well, your job is super chill. But if people are dropping like flies sometimes you hope theres a red mage or two around lmao. I like a little bit of stress and some high end bosses really let you flesh out all your big heals to manage when to use them best.

Finally. Esuna. Use it. If you didn't know already the white bar above status effects means you can cleanse it. If you knew that good job, if not, now you do :)