It seems, though, that more players beyond the original hacker are starting to manipulate data in Splatoon to play as Octolings and use the Hydra Splatling. Unfortunately some are reporting that if they end up in a lobby with this particular hacker - reportedly not the same one that first showed playable Octolings - it can cause issues with their save data; if you quit at the wrong time and the game is trying to populate this unofficial character in your plaza it can cause the game to lock and corrupt data.
Splatoon players have been chatting about this on various social media and message boards, with the following summary from Reddit being useful.
This player can soft-lock your save, destroying all your hard work and making you restart the entire game with a blank slate; if you don't take the proper precautions. Don't let this happen to you.
-Tips to be safe around this player:-
Backup your save! Copy it to a USB. Do this now, before you get a chance to see them. If you see them and you lose connection in a match with them, it's already too late. Here's the official Nintendo Support page with instructions on how to do this. If that doesn't help, here's an alternate guide on GameFAQs.
If you spot them, do NOT leave the room! Keep playing until they're no longer in the match. This stops your plaza from trying to load them, which is what destroys your save. Alternatively, you could back out of that room and immediately join another Turf War, but do not return to Booyah Base.
If you are too late, all hope is not lost. But. You'll be waiting until the next Splatfest. I personally do not know this for sure, but presumably loading up the game during Splatfest will wipe the player out of Booyah Base for you, thus freeing your save from the soft-lock.
If you see this player, please take down their name and report it. The closest info we have so far is that "the name started with a k and had numbers in it".
If you want to help spread the word, go write a post on Miiverse about it. Write it in drawing form, so it can spread around as graffiti as well as people seeing you.
Hopefully this issue is minor, with relatively small numbers encountering this issue online. As awareness spreads it's also more likely to prompt an update from Nintendo to block this particular problem.
Have you encountered 'Octoling' mods online? The tentacle hair is the key giveaway, it seems - let us know and keep those eyes peeled.