Hello. Sadly, that's most likely not residue. That's probably your plastic melting. You definitely shouldn't continue to use that particular
juice with your tank. Let me guess. It's either cinnamon, menthol, or citrus flavored. Cinnamon flavors as well as other highly acidic flavors such as strong lemon, other citrus, and heavy menthol flavors are known to damage the plastic of tanks and clearomizers.
Do you know if you're using tanks made with polypropylene plastic? They supposedly hold up much better with acidic juices.