Marimo moss balls sold at pet stores have been found to be contaminated with Zebra mussels.

Invasive zebra mussels found hiding in aquarium plants for sale at pet stores - Natural Resources

Invasive mussel triggers widespread talks, increased coordination with pet stores

SCDNR warns of zebra mussels sold at local pet stores, News

UPDATE to Consumer Alert: Aquarium Moss Balls May Contain Invasive Zebra Mussels - N.C. Wildlife Resources Commission

Invasive Zebra Mussels Detected In Aquarium Moss Balls

Zebra mussels enter waterways via moss balls in home aquariums

Invasive Zebra Mussels Reported in Marimo Moss Balls - AS Magazine

Zebra mussel - Wikipedia

Pest alert: Invasive zebra mussels arrive in Oregon in moss balls sold for aquariums

Beware of Infected Moss Balls Recalled From Utica/Rome Pet Stores

Invasive Zebra Mussels Reported in Marimo Moss Balls - AS Magazine