How to seal the exposed top of glass block windows
Buy some anchors and attach them a good few inches up on each side, and across the blocks.  Throw some mortar or a generous amount of glass block adhesive on top of the anchor (you can also screw it into the joists).  Throw a 2x4 firmly on top of mortar so your gap is equivalent to your other gaps.  Then secure the 2x4 (might even think about 2x6 for this installation) to the joists with a few screws on each side.  Caulk any corners or gaps.  

This will finish your install and keep the air leak issues.  As it stands your blocks are kind of dangerous right now.  That middle one isn't secured and when these things fall and break... they explode and shards could be in person they explode on.  The anchor and top plate will fix this issue and help fix your air issue too.  (If you still notice any drafting after caulking I suggest smashing some roxul into the area.