One of the admin accounts appears to have been compromised. The owner/other admins appear to be aware now because that account had its admin access revoked and offending posts are being removed.
Definitely opens up a big question about the security of Lemmy instances that I am sure will be discussed over the next few days.
More time will definitely be needed. I'm glad they caught it and acted quickly enough to prevent more vandalism from occurring, but until we know how the account was compromised and what else they may have gotten in the process, it's still a situation to keep an eye on.
Not a whole lot - you might see some spam being federated from lemmy.world but I'd expect the lemmy.ml and lemmy.world admins will fix it, and them clean it up.
That's probably good stress test to figure out how to handle that.