He expressed extremely hateful views on Caucasians, gays, immigrants, and Jews in his blog posts and videos around 2010.
He pushed "the great replacement" conspiracy theory right in 2017 saying that Putin conducts Islamization of Russia (his article on why Trump is the opposite of Putin).
All these cases make him controversial for the modern world where people are "cancelled" for much less.
As for hateful views on immigrants, well, they boil down to that Russia needs a visa regime with certain middle asian countries. To note, every single EU state, US, Canada, Australia, etc DO have visa regime with those states, and with Russia. Apparently, all politicians in these countries who do not act to abolish the visas are hateful extremists.
Navalny is not angel and I do have many questions for him, yet, I feel you are mostly reading propaganda websites.
About migrants compared to rotten tooth - from his official youtube - https://www.youtube.com/watch?v=ICoc2VmGdfw
BTW, Looking at your previous comments I can see that you only post about Russia. That's red flag.