It's been said before; we had a huge discussion about this a month ago but never the less I will answer your question.
The symbol they wear on their belts can be of significance but we don't know for sure.
However it does represent both the heartless and nobody symbol (more likely used as an easter egg) but that doesn't mean that Maleficent is there master in the least.
For one the heartless didn't exist before bbs so the symbol would be invalid if it were based on the heartless which Maleficent controlled afterward. Terra and Maleficent do have a relationship but not as master and apprentice. It is said that he's slipping into darkness with the help of Malef in some way or another; she's possibly seeking the "beings of pure light" something MX is after as well. So with MX after them Terra sees that darkness might be the only possible way of finding him.
And lastly, their master is male
It's ok though, you're not crazy, honestly it's the most logical thing I've heard all week when it comes to theories.