The most insidious thing about racism on Mastodon is how it is done in such a way that the majority-white user base never sees it. I don't think that it's simply a matter of white people not seeing what they don't want to see - at least in the vast majority of cases.
So when a non-white person discusses the racism they see as a user of Mastodon, it must feel like gaslighting when white people respond with "What do you mean? I've not seen anything like that here."