We're all gonna need this sooner than expected so here is my proposed APA 7 style citation format for citing a Mastodon post

Lastname, F.M. [@mastodon@handle.tld] (20XX, Month DD). Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut ... [Mastodon post]. Name of instance. [URL of post]

As with the format for citing a Twitter post, the correct reference list title is the first 20 words of the post, and the correct short title is the first 5, both using an ellipsis if necessary.