How old is the tank? What’s ur water change schedule? Sounds like a bacterial bloom. Maybe a mini cycle. I’d do a big water change at least 50% twice a week and start today. I’d add a dose of seachem prime now and with every water change to control the water.
The tank he was in before with my other goldfish was bought earlier this year. I just put him in this tank to quarantine him. I’ve had it around the same time as the bigger one. I do big water changes twice a month and small weekly water changes.

