While this does not answer your question, I believe it may help you with an alternative approach, which won't hide it completely, but will make it a lot harder to figure out.
Rather than keeping the password in plain text (which will be easy to spot in a hex dump), you can hide it somewhat. For example, exclusive-or each byte of the password with a randomly chosen number, put the result in your code; then, when you want to log in to the wifi, exlusive-or it with the same numbers (which must also be on your code).
Note that all the information required MUST be on the Teensy, so it is not impossible to reverse engineer; however, it will be hidden from casual inspection.
Note that, although more complex obfustication is possible (using any off-the-shelf encryption), since both the code and any keys will be available to an attacker, it is a token gesture only.
The Teensy v3 has JTAG, which means a running Teensy can be inspected, single-stepped, all memory and pin states can be viewed & modified. Once someone has your device, it's pretty much a lost cause against a determined attacker.