It could be nothing more than a bit of injury damage to the tip of the cotyledon. Given the true leaf development they will soon begin to shrivel and die anyway.

Or in soil grown plants it can indicate nitrogen burn or salt damage. In hydro growing, I can't say as it is a whole different world. But you could probably get some more accurate info over on the Hydroponics forum here.

Dave

Awesome thanks for your feedback! A few friends I spoke to mentioned that its normal for them to die once true leaves develop :)