Decay only turns mobs green, 0 damage?

So I spent the Cs to get two essences of delirium and used them on an axe and a staff. Now, however, when I try to use both of these weapons the decay aspect only turns mobs green with no damage done to them. What is the deal with this?

I first noticed the problem on mobs that were resistant to chaos. I then tested on mobs that weren't and it did no damage on either of them. Do hits need to crit for the decay to take effect? Shouldn't it state that if it's the case?

How can you use both an axe and a staff? Are you weapon-swapping them?