The icon for the Bad Luck effect.
Bad Luck is a status effect which decreases the probability of finding high-quality loot from fishing or naturally-generated chests. Instead, it increases the chance of junk fishing loot, junk in treasure, and bad ores rather than iron.
Source[]
This status effect can be gained from one source:
/effectcommand
Opposed effect[]
There is a Luck status effect, which functions in the opposite manner as "bad luck". This positive effect increases the chance that a player will find high-quality loot. There are various levels of this status effect; The higher the effect level, the higher the quality of loot.