What do they call a Brownie troop leader?

There really is no different name for leaders of different levels of Girl Scouts, they're all just called "Leaders".

In addition:

In many countries, the Brownie Girl Guide troop leader is called "Brown Owl" after the Wise Old Owl in "The Brownie Story" who helped the girl find the brownie (herself).

Girl Guide leaders are also called "Guiders".