I'm not a banana expert by any stretch but I've seen little spots on mine on occasion and as long as they're growing I ignore them. Mine wipe off so perhaps it's been a surface fungus or scale of sorts.

I have several Brazilian bananas that grow like yours, one leaf a week, very little vertical growth so I'd have no worries there.

Anyway, are you a member of bananas.org? They could be a better resource than garden web for banana issues... lots more traffic relating to bananas.

Regards,

Gary