What mussels actually eat is algae and other microbes in the water. This isn’t “dirty”, it’s just cells. In the process, mussels may also filter out sediment particles, but they aren’t actually incorporating those into the body, it’s just being excreted as waste.
Mussels can definitely accumulate toxins, especially during harmful algal blooms when toxic algae are present. That’s why they close the fishery at those times. And they can absorb pollutants if they live in polluted water. But other than that, they aren’t particularly likely to contain anything toxic.
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