The poem opens with a question, or at least a wish: who will see the divinity of slugs? Slugs are usually things people flick off lettuce or dread stepping on, so calling them divine is a playful, surprising move. The speaker seems to want someone to look properly, to see slugs as "full," not "plain," meaning rich and complete rather than dull and slimy.
The middle lines ask for a kind of transformation in the eye of the beholder. Slugs should look like "velvet and lace," fabrics tied to luxury and delicacy. Nothing about the slug changes; only the way we see it does.
Then the last line tips the poem into humor: "get them for supper." It's unclear whether this means eating them or simply welcoming them as guests, and that ambiguity is part of the fun. Either way, the tone is light, cheeky, and a little absurd. The mood is curious and affectionate, with a wink. The speaker seems to enjoy how reverence and ridiculousness can sit side by side, and invites us to look again at something we usually dismiss.