So it’s a pretty well known fact that Michael Jackson and Dustin Hoffman did uncredited voices on the Simpsons.
I know who M-Jack was: The prisoner who thought he was Michael Jackson, also choreographed the Bart dance.
But I can’t for the life of me get who Hoffman was. Anyone?
Mr Bergstrun (aka Mr Boogerstrum), in the season 2 episode, Lisa’s Substitute. He was credited as “Sam Etic” because Dustin had concerns that doing a cartoon would hurt his reputation (this was before there were many big stars doing the show - he was the biggest one at that point). The quick callback to The Graduate in the episode should have cast away all doubt that it was him.
IIRC, it was an episode where he plays a substitute on whom Lisa develops a crush. Can’t remember what it was called. The Substitute, maybe?
Oooohhh neat.
I haven’t seen the episode so that’s probably a big reason. I’ll keep an eye out for it. Thanks!
He was Mr. Bergstrom, and the name of the episode was Lisa’s Substitute. Back then you could sometimes figure out the content of the episode from the title. 
fusoya is right about why he used a pseudonym. Albert Brooks was credited as “A. Brooks” for that reason - and he still uses that credit when he’s on the show, which is not as often as it used to be. Michael Jackson used the name John Jay Smith. But the really odd part is that he doesn’t do the singing in that episode.