I would just stick with cheato. Calpura can release spores into the water and take over the tank.
Is it gently tumbling or is it just a big mass of algae? If it isn't tumbling, you will need to manually rotate it every couple days. Otherwise it starves itself of light.
What are you parameters? You might not have enough extra nutrients in you system to support it. But with your stock list, I doubt this is the case.