Toa isn’t an Agori word, so when the Toa speak Agori, it likely comes out as the closest equivalent; Master being a good fit.
Spherus Magna had 6 distinct regions, one of which is the Great Jungle. Even if we assume that the particular serial is canon, Lewa was last seen on Bota Magna/The Great Jungle, and if he’s been there for a while, it would make sense that he would be known as a Master of the Jungle, and not just for his Elemental Powers.
I love this explanation. It makes so much sense!