Do you mean tiny bubbles that latch onto the plastic tube?
If so, they are completely normal and will go away in time... Warm water holds les oxygen, therefore as the water nearest the heater warms, it's ability to retain oxygen/air decreases causing the oxygen/air to form bubbles. Until the heater builds a "slim coat" the bubbles stick to the heater.
If you mean the plastic is bubbling...
This is a manfunction and you shuold contact the manucaturer to arrange an exchange (they have a lifetime warranttee).