It looks like a parasitic infection of some sort. Jungle Parasite cure is a good choice. Convicts are pretty hardy, but difficult water conditions will overcome even them. Wren is right about the water change, even though the water has evaporated, it has left waste products behind when it does so. These wastes are what build up and can cause health problems for the fish. To change the water you'll need to actually remove part of the water in the tank and then replace it with new dechlorinated water. At this point, since the fish has been living in these conditions for a while, it has adjusted to the level of dissolved solids in the water and will need time to adapt back to a healthier water quality. I recommend doing 10% weekly water changes (remove 10% of the water then fill the tank) for the next three to four weeks, then increase to 15-20% once per week. That way the fish's osmotic balance isn't tanken for a ride, and it won't be shocked by a sudden change.
Hope this helps!
