For my Russian I have a small dollar store humiditier but if the enclosure is in a big room you might need a big humidifier or there are products that shoot water into the tank at times of the day and that will raise the humidity
Flex seal is toxic I believe but if it the tape or at the bottom covered with substrate or outside the enclosure it should be fine but if keep an eye out if (s)he gets to close to it and might think it's good it could happen but unlikely
If it's not a crack or something like that then I would wait and see what happens it might just be a "birthmark" or birth defect but if it's a crack I'd talk to a vet because it could get infected and that's by all the organs