Well, this experience was short-lived: just received this email from Petco:
"At Petco, weíre committed to supporting the health and wellness of pets, people and our planet.
We were recently approached by the Washington Department of Fish and Wildlife regarding an investigation into a potential link between Marimo aquarium moss balls and an invasive species of wild zebra mussels. To aid in the investigation and prevent potential spread, we immediately paused the sale of all Marimo aquarium moss balls. We are working closely with regulatory authorities, Petco vendors and our own veterinary staff on appropriate next steps, including proper handling and disposal of any affected product.
If you have Marimo moss balls at home, please safely and responsibly dispose of them by following these steps from the Washington Department of Fish and Wildlife. If you purchased Marimo moss balls from Petco in the last 30 days, follow these steps and then bring your receipt to a Petco location to receive a full refund. "
I was also told all moss balls were removed from store and sale was prohibited until further notice.