Why does he have heat on both sides? What type of thermostats? I would plug in only one side into one thermostat then plug that thermostat into the second one as back up? Either that or unplug them both and keep the room above 70 F. A ball has to sit in disgusting wet substrate for quite some time to get scale rot so I doubt a spilled water dish once in a while alone would do it. Mine spill water a lot, sit in wet tubs for a couple of days while breeding (they usually will empty out before copulation), etc. and I have never had a single issue with rot of any kind. I would think it is a burn caused by a malfunction in one of your heat sources thermostats. Soaking in water may be because he is way to hot.