Thanks. There’s no “Remove account” button for this account, though there is for some others. I think it’s because it’s a Microsoft account which was set up in the “Online accounts” section of “Settings”. I wanted to remove it because it wasn’t working.
I tried removing the Microsoft account from there and adding it again - this more or less worked, but Geary now lists two Microsoft accounts, one of which has a warning triangle against it. However, there is just one mailbox for the Microsoft account, and it works, so it’s basically OK. The spurious entry in the Geary accounts list doesn’t seem to be doing any harm.