It's rough, getting kicked out of the Garden of Eden, and Herbert uses the language of material hardship to describe it. Adam is created with "wealth and store" but once he makes his fatal error, he loses all of it, going from poor to "most poor." In Herbert's version, Adam is like a careless millionaire who starts out with a great house and pool but after making unwise investments falls into foreclosure and bankruptcy. Talk about a recession.
But the word "decaying"—a surprising choice that doesn't fit in with the other images of material wealth—signals that this poverty might also be a little less literal. Just take the Garden of Eden, which is both physically abundant as well as symbolic of God's abundant love. In the same way, Adam's loss of his material goods = the loss of his spiritual closeness to God. Both types of poverty are made visual in the length of line 5, which clocks in at just two iambs.
Lines 1-2: Adam's rich for exactly one line before it all comes crashing down. Although Herbert could have opted for a lot of different imagery to describe Adam's creation, he goes for "wealth and store," encapsulating the abundance of Eden life with this metaphor of material wellbeing. The actual Garden was like Monopoly: no real money.
Line 3: "Decaying" is kind of a weird choice here, one that doesn't really fit into the overall analogy of riches-to-rags. Bananas, teeth, and corpses decay, not money. But remember that "wealth" is standing in here for a general sense of abundance, both material and spiritual, and that is definitely subject to erosion over time. The main point is that Adam's losing it and losing it fast.
Lines 4-5: Herbert continues the wealth metaphor to line 5, where Adam's lowest point is described in terms of poverty. From the well set-up life of line 1, he's fallen to this.