These are not full aluminum engines. They use different types of metal for rigidity. Haven't you ever heard of aluminum alloy? This is nothing to worry about. So, if the whole engine is made up of "Aluminum" then why would you have a warped head, and then once you check the deck on the short block, there is no sign of light on the other side of the straight edge? Just saying, "If I was the mechanic, I would not do anything about it." I can also promise you the Toyota dealer is going to say the same thing. There is nothing to worry about. You aren't loosing any oil. That looks like nothing other then a little bit of PCV residue. If it was leaking 1. You would clearly see it. 2. You would more then likely have a Lean code setting off the check engine light.
Lean or Rich codes are for too little ( Lean ) or too much ( Rich ) fuel to air ratio.