Does decaying touch cause damage when elementally attacked, with elemental resistance > 100%?

Decaying touch skill:

Make a target decay, so that it cannot be healed by magic or potions anymore, it will be damaged instead.

Say there's a monster with 200% fire resistance. Casting fire spells on it would heal it by default.

If it is under the effect of decaying touch, would it now be damaged instead, when fire spells are cast on it?

This will not heal an enemy that gains health from elemental magic. Enemies that gain health because you attack them with spells that match their type are not the same as a healing spell/food/potions.

Decaying touch is only if you use regeneration spells, healing potions, or consume food (basically all the ways to heal yourself). Those now damage you (or whoever has decaying touch status).

In Divinity Original Sin 2, the Decaying status makes it so any healing done to the afflicted character will instead take damage. This means not only will regeneration on a human character takes damage, but poison on an undead character also takes damage as that's their normal means of healing.