It takes time, haha. Units heal at Harmony level 6, as said. Harmony ground uniques see a lot of in-miasma strength bonus perks after level 11. So miasmic terrain strengthens your units and hurts other affinity units simultaneously. (Or you can choose more universal perks for your units if you anticipate fighting another Harmony faction more).
Miasma satellites are easy to throw around so feel free to remove miasma from your tiles for now and add it back in late-game.