I doubt anyone at Facebook is freaking out about Mastodon and setting out to censor all mention of it. It's probably just a keyword or link that tends to co-appear with other, actually rule-breaking content, and some automated system has learned to block it. Still, it does seem like the sort of thing that could get Facebook in trouble with a regulator if you squint at it.
Perhaps after certain number of people report it, it automatically will be marked as one (such logic would be more reliable on FB, than for example reddit or HN, since most FB users have a single profile tied using their real name)