Using whey from yogurt to make ricotta
Yogurt whey cannot be used to make ricotta.

With most cheeses, including mozzarella, the milk isn't boiled. The casein proteins are bound up with some of the lactose and almost all the fat to make the curd. The whey for such cheeses contains the rest of the lactose, tons of vitamin B, and almost all the albumin.

The albumin proteins are water soluble when they haven't been denatured. They also denature at a higher temperature than the casein does. Therefore, they almost all wash out of the curd with the whey. Bringing mozzarella whey to a (almost) boil denatures the albumin which precipitates out and can be strained to make ricotta.

When making yogurt the milk is heated to 190 or higher and then cooled. This is precisely to denature the albumin. The extra protein gives the yogurt a lot more structure. 

However it means that there is no protein left in the whey- mostly lactose, lactic acid, and vitamin B.