The more usual Greek word seems to have been polypous (also pōlyps), from polys “many” + pous, but for this word Thompson suggests folk-etymology and a non-Hellenic origin.
The classically correct Greek plural (had the word been used in this sense in ancient Greek) would be octopodes.
Octopi regards the -us in this word as the Latin noun ending that takes -i in plural. Like many modern scientific names of creatures, it was formed in Modern Latin from Greek elements, so it might be allowed to partake of Latin grammar in forming the plural.
Still I’d prefer octopi since despite origins in different languages its now a word in english and we can use it however english speakers like and not the greek.