The damage maybe worst than what you think or expect. If leak is discovered early before it dry up, chances wiping away may help. The longer it is, chances that it has damaged the PCB boards inside which will be extremely expensive to replace - as good as getting a new one from my recent unfortunate experience.
if it had already happened, it is almost impossible to clean. the liquid had gone through any small openings to inside the flash.
it is better to avoid this situation by always took out the battery after usage. never ever store the flash with the batteries inside.
i have 2 very bad experiences with battery leakage for my sb-25 and sb-600. both are still able to fire the flash, but no control can be done since the electronic circuit inside the flash was malfunctioning.
There is nothing left over to clean if a Lithium battery leaks. Do you remember the stories about exploded hp batteries "Made in China"? - That's what happens if Lithium gets in touch with Oxygen of the air.