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The sentence is "諸星先生は元々住んでる星が違うし". This looks to me an idiomatic expression, but I cannot find this in dictionary.
Another interpretation I was thinking about is that the teacher in the above sentence "lives in another star/solar system", which is to say that it's just a completely different person (in character). It fits, but not sure if it is accurate; for one, living in another planet feels more accurate, and if so 星 should be short for 惑星？
「諸星先生{もろほしせんせい}は元々住{もともとす}んでる星{ほし}が違{ちが}うし・・」
While 「住んでる星が違う」 is certainly an exaggerated and/or metaphorical expression, I would not necessarily call it an idiomatic expression because it means what it literally means, which is "to live on another planet" as you said.
「星」 can mean "a planet" as well all by itself.
"Mr./Ms. Morohoshi lives on another planet to begin with."
A super-literal TL would be:
"As for Mr./Ms. Morohoshi, the planet s/he originally lives on is different."
This is a good example of how literal translation rarely works well between the two languages. What I am trying to say is that if you translated the very natural-sounding English phrase "to live on another planet" directly into Japanese, you would not arrive at 「住んでる星が違う」, which is a completely natural-sounding Japanese phrase.
There isn't any context, so I can't be 100% sure, but the super short answer is probably that the speaker is trying to say that Mr./Ms. Morohoshi has a completely different set of values than those around him, which makes him stand out and appear quirky or possibly mildly insane.
