stuck on blank square

There is a blank tile in the upper left corned, highlighted or whatever you call the tiles that have to be hit twice or unlocked. How do I get past that? I've matched up all of the other gray squares so there's nothing left to do and I don't understand what I'm supposed to do to get the level to finish. There's also a blank tile 2 tiles underneath the blank gray tile but I don't think that one needs to be dealt with. HELP!