is it on the silicon or in the silicon?
try scraping it off and see what happens.
if it's bubbles in the silicon then the silicon is dodgy and the tank should be swapped for another one.
check to make sure it's not reflection from the gravel or polystyrene foam under the tank (assuming there is any).
Thanks for the advice.It's probably just algae. I wouldn't worry too much if it's on the silicon. But if you ever get bubbles in the silicon, then it's time to worry.