That is from the Woodrot Warden, a DLC boss. That status should have gone away after the fight, win or lose. You said you aren't "using any mods which would add the boss", but does that mean you are using mods? it's possible that a mod may have glitched up the removal of that status.
Did you actually beat the boss yet? If not, try beating the boss without any mods and see if the status goes away. If you already beat the boss, try reverting to a save before fighting the boss and try fighting the boss without any mods. If the status goes away after the fight like it should, it was a mod that caused the issue.
