What is causing the minor leak in flex supply line under sink?
The joint that you are pointing to in your picture is a compression joint. Usually a little bit more tightening of the nut will stop it.  If you are absolutely sure you can't get another 1/8 or 1/4 turn on the nut, then simply shut off the valve and replace the hose.  The Teflon tape mentioned in the previous answer is necessary on new hose threads, but has nothing to do with a leak in the joint you are leaking from.