I did a quick playtest with a Necromancer/Polymorph hibrid made from a reptile cleric base and it turns out that Decaying Touch also messes with the healing done to undead from poison sources (test dumy used was the fresly burried corpse on the beach).

If I could just find some arena matches to test Decaying Touch vs healing from necro and mosquito swarm I could better determine the value of this skill in PvP and in future mob compositions.