Grey Smudge On Sterbra Cory ?

I'm not really sure what it is. Does the cory behave normally - eating, pottering around like the others? If it is I wouldn't worry just yet as sick fish usually go off their food and sit in a corner with clamped fins.
The only possibility to watch for is fungus, which is actually a bacterial infection. If the greyness starts to get worse you will need to treat with an anti fungus and finrot medication. But you can't use it in the main tank because of your shrimps; you will need a quarantine tank to use any medication.

Myxazin is one of the two meds I would have suggested. Fingers crossed.

Ok I didn't know that, thanks Extreme one for info.