So we've already talked about pluralizing genus names and how you DON'T DO THAT, but what about possessivizing? Have we already discussed that? Is "Anubias'/Anubias's tendency to flower underwater for everyone except fishmostly has inspired an entire new tag to document her annoyance" grammatically correct? If not, how would one word it? Something like "the tendency of Anubias species..." is probably safer/more professional, but hypothetically.

Nope, can’t do it. Actually, it’s basically covered in the original Plurals and Possessives in Taxonomic Nomenclature post, albeit briefly. Formal ranks, including genera and species names, cannot be made possessive.