Get the batteries out and wipe the electronics with a soft towel soaked with a little vinegar to neutralize the corrosion. Then a damp towel with just water to get the vinegar up. Let it fully dry before putting in new batteries. This happens to batteries that have sat unused for a long time installed. It can also happen if at the end of charge, you pull the batteries out, and put them back in to squeeze a little more use out of the batteries. Do it too many times and there is a leak risk. Cheaper batteries do it more often but the top brands will occasionally leak as well.
Get the batteries out and wipe the electronics with a soft towel soaked with a little vinegar to neutralize the corrosion. Then a damp towel with just water to get the vinegar up. Let it fully dry before putting in new batteries. This happens to batteries that have sat unused for a long time installed. It can also happen if at the end of charge, you pull the batteries out, and put them back in to squeeze a little more use out of the batteries. Do it too many times and there is a leak risk. Cheaper batteries do it more often but the top brands will occasionally leak as well.
Vinegar here really does the trick in neutralizing the acid. do not skip it!