Question

If Kecleon got Nature Power and it had Protean, what would its type become outside of wifi?

Answer 1

When using a move that calls another move (such as Nature Power), the type of the called move will be considered.

Protean (Bulbapedia)

I tested this in Showdown, which backed up what Bulbapedia says. I think Showdown just had this interaction coded wrong when Sempi tested it.

Answer 2

What seems to happen for me is that Kecleon's type REMAINS Normal
As shown by this replay - I made Kecleon utilise Grassy Terrain, which changes Nature Power's attack to Energy Ball. However, Kecleon's typing REMAINED as Normal type.

I would guess this is because;
The opposing Kecleon used Nature Power!
Nature Power turned into Energy Ball!

Since Nature Power itself is classified as a Normal Move, and chances AFTER Kecleon uses the move (As shown by the text), it takes the first typing which is Normal and not the Grass typing of Energy Ball.