If you have had this new iMac for longer than 14 days, you can no longer return it for a replacement.
You might be able to get a new, replacement LCD panel installed under AppleCare, if this bothers you or you are just going to have to live with it.
There is a certain amount of dead pixels that determines if the LCD is good or defective.
Every LCD panel is prone to dead pixels. The pixel can be stuck "on" (white) or stuck "off" (black).
If this is the only pixel that you found that is dead, then it maybe considered OK and not defective.
If you had the LCD replaced, there is no guarantee you wouldn't get a replacement LCD with another or a few stuck pixels.
In the near future or before your free year of AppleCare expires, I would strongly consider budgeting, purchasing and registering with Apple for the 3-year extended AppleCare warranty plan. You will need it.