The recently highlighted issues concern only LIVE zebra mussels, which have been classified as invasive in the USA only – there hasn’t yet been a live one reported in the UK, nor any issues of invasiveness, and it’s only live ones that are cause for concern. In the extremely unlikely event that you do ever find a live one, report it to OATA or DEFRA in the UK. Certainly don’t be alarmed if you ever find a dead one, its completely harmless. Moss balls are not banned in the UK.
We use a highly reputable supplier in Holland, who grows all moss balls in a captive environment to minimise the risk. To further reassure our customers, when the moss balls arrive here we freeze them for two hours prior to release, as advised by OATA. This isn’t harmful to the plant, but kills any hitchhiker, including zebra mussels, which may have found its way onto the plant. You can repeat this if desired.