Just saw this on CBC:
https://www.cbc.ca/news/canada/calgary/moss-balls-zebra-mussels-environment-alberta-1.5944959
Apparently some moss balls imported into Alberta may contain zebra mussels. If you did buy moss balls this year, don't flush, dump or compost them, the government says. Boil or freeze them then put into the trash.
Excellent, thank you. I wish more people cared about this.