Bonta looks at a slug and, instead of recoiling, dresses it up in glamorous language. In the first two lines the slug becomes "light lined with velvet," a soft, glowing thing, and a "modish rider on foot," a stylish traveler who somehow gets around on its own body. The joke is that something slimy and slow is treated like a fashionable figure.
The second stanza pushes the humor further. The slug is "full of beer," which suggests the old garden habit of luring slugs with beer, though the poem never says so outright. Here it is a "prodigal memorandum," a wasteful, indulgent little note "signed and sealed / on the arithmetic of pleasure." The slug's life seems reduced to a tidy sum: indulgence, then the bill.
The speaker sounds amused and oddly admiring. The mood is playful and a little tipsy, with a hint of darkness under the wit, since the pleasure the slug enjoys may be the thing that undoes it.