Game concept
County Corruption
From the game's encyclopedia
While a County has low Control or its Holder is in Debt, the County might acquire County Corruption. These are various kinds of negative County Modifiers.
County Corruption will fade over time, but if the root cause persists (Debt or low Control), it may reappear.
The Marshal's Increase Control in County Councillor Task can clear out County Corruption faster.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
County Corruption is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "While a County has low Control or its Holder is in Debt, the County might acquire County Corruption." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.