brew update and
brew upgrade, Sequel Pro suddenly stopped connecting to localhost, bombing without much explanation. Several google searches later, I found this ticket which mentioned running mysql_upgrade.
It’s possible I missed a homebrew notice about this, as it updated a ton of packages, but this ended up solving the issue:
mysql_upgrade -u root -p
Followed by restarting mysql:
launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.mysql.plist
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.mysql.plist