If the stalk of the bamboo in your tank turns yellow, it's most likely that the bamboo has died.
For the safety of the BioSphere, you want to remove the bamboo immediately. The bamboo can be easily removed from the tank by gently pulling it straight up.
While the bamboo is a part of the Nitrogen Cycle, it is not necessary for the frogs health. It does, though, add to beauty of their habitat!
You can replace it with any 4 inch bamboo - that you can typically find at a local nursery. To add it back, push the bamboo down into the gravel/sand while slightly spinning it. Once it has hit the bottom of the tank, gently push the decorative rocks around the bamboo to hold it in place - while the roots grow into the colored gravel and sand. It's okay if some of the bamboo's roots are exposed.