The poison of the Honey Bee Is the Artist's Jealousy.
An artist is like a honeybee because they both create sweet, good things: art and honey. So the sting or poison of a honeybee is like the artist's dark side. Although artists are doing something nice, they can't help getting jealous about other artists.
There are actually two ways you can take this couplet, and they both make a lot of sense (though one might make a little more). If the "poison" of the bee is a reference to its sting, then it means the artist's jealousy—which can be pride over his/her work, or jealousy of someone else's work—is something that can either defend that artist or injure other people.
But if it's not a reference to the sting, and it's a poison that actually poisons the bee and wrecks its ability to create honey, then Blake means that jealousy is something that ruins the artist's ability to work. Both make sense, but maybe we lean toward that first interpretation.
The Prince's Robes & Beggars' Rags Are Toadstools on the Miser's Bags.
This is another straight-up metaphor. The robes and rags "are" the toadstools. And this couplet doesn't have anything to do with animals.
Also, this one is pretty tough. We get that Blake is saying something negative about the prince's robes and beggar's rags, since he's saying that they're toadstools. And being a miser isn't usually a good thing. But in what sense are the robes and rags really like toadstools on a miser's bag? It's sort of a puzzle, though it definitely has a solution.
First of all: princes are rich and misers are poor. Yet both of their kinds of clothing are being compared to toadstools—fungus. And where's that fungus growing? It's growing on a miser's bags—the bags of someone who hoards money and doesn't use it, especially not for charity.
So, Blake might be saying that the miserliness of human beings—their greed and attachment to money instead of what money can be used for—helps cause inequality. Some people are wearing robes, and others are wearing rags. If people had a more generous spirit and saw themselves as part of a greater human family, this probably wouldn't be happening.