if the compression nut is tight, then you should not have a leak. dont put teflon on a compression joint face. you can, if desperate pull it apart and coat everything with a good layer of pipe dope and then reassemble. sometimes you may have a little corroded pore that this can fix. might be time to replace the supply line, nut and ferrule. other possibilities:
1) its not leaking from the supply connection. in these setups, its sometimes hard to diagnose where the leak is due to access issues. check the compression nut on the cross connection. they often work themselves loose and cause a tiny leak that wicks down the threads of the valve body and makes it appear the leak is coming from the supply line connection.
2) valve body itself could be cracked or leaking from one of the valve bushing nuts up above. same logic as 1)