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.