The balsa core has got wet and has rotted. Before they get so weak that they break when someone steps on them, you need to repair them. It's easy to do. Just remove them, flip them over and cut the skin off that is encapsulating the balsa. Remove all the rotten balsa including the discolored portions. The discolored part is mold in the early stages and if left in there it will continue to grow. Replace the core with the material of your choice and reseal the whole thing using fiberglass resin and mat. You can find threads on this forum with pictures and discussions about all the particulars.
