The sky-like limpid eyes, The circular infant's face, The stiffness from spats to collar Never relaxing into grace;
At first glance, it's almost impossible to tell what Pound means by "Brennbaum," which seems to be the name of the character he's talking about in this next section. It seems to be a name that Pound just made up, like Monsieur Verog. But if anything, the name is probably a half-racist portrait of Max Beerbohm, who Pound thought was Jewish. The funny thing is that Beerbohm wasn't even Jewish.
If you check out a pic of Beerbohm, you can sort of see a resemblance to Pound's description. You can picture those eyes of his being really blue, and yeah, they do look a little limpid or half-sad. But Pound doesn't like how stiff this guy is "from spats to collar." We all know what a shirt collar is, and spats is just an old timey word for socks. So Pound's saying that from top to bottom, this guy's a big stiff.
For Pound, the fact that this Brennbaum guy can never really be graceful is because he can never relax. In fact, that seems to be Pound's main beef with most of the people he mentions in this poem. They're all trying too hard to be prim and proper, and they need to connect with their deeper passion (which just makes you wonder how Pound could've been friends with a puritan like T.S. Eliot).
The heavy memories of Horeb, Sinai and the forty years, Showed only when the daylight fell Level across the face Of Brennbaum "The Impeccable."
Now Pound looks like he's talking about "heavy memories" that you can see on Brennbaum's face in full daylight. Whatever these memories are, they seem to be connected to Horeb, Sinai, and the forty years, which are references to Mount Horeb (also called Sinai) where the Jewish leader Moses got the giant stone tablets that laid out God's laws (especially the ten commandments). The forty years refers to the forty years that the Jewish people had to live in the wilderness after they left Egypt to escape slavery.
Now here, Pound seems to be acknowledging all of the hardship the Jewish people have had to deal with throughout history.
But when he calls Brennbaum "The Impeccable" in line 145, the quotation marks might suggest that he's being sarcastic. He might be saying that all that stuff happened thousands of years ago, and that it's no excuse for a Jewish person to act all hard done by (remember, the Holocaust hadn't happened at this point). So basically, Pound seems to think that Jewish people are especially bad at acting like posers and trying to hide their passion so they can always look like they're in control.