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This normally indicates that the plant either is not getting enough light or enough water. If you can, get them more light and increase water a little.

As long as they still feel firm and not soft, they will be viable for planting.

You may be overwatering it. This article should help determine if this is the case: https://www.gardeningknowhow.com/plant-problems/environmental/signs-of-plants-affected-by-too-much-water.htm

Yes, you can. Caladiums are popular houseplants as well as indoor plants, so they would do well in a greenhouse. You just need to make sure thay stay watered and that there is relatively high humidity, which should be easy in a greenhouse.

Since caladiums are considered tender perennials, they must be dug up in the fall and stored indoors over winter in cold climates like yours. Once their foliage yellows and begins falling over, caladiums can be carefully lifted from the ground. Place the plants in a warm, dry location for at least a couple weeks to dry out. Then, cut off the foliage, place the tubers in a netted bag or box, and cover in dry peat moss. Store the tubers in a cool, dry location. Once spring returns, you can replant outdoors. If you are growing caladiums in containers, they can be overwintered indoors.

This article will help you:
http://www.hort.purdue.edu/ext/HO-085.pdf