If Mars' moons were captured wouldn't that indicate that smaller rocks have also been captured by Mars?
I think a simplier explanation is the moons were formed from impact ejecta. Think of it this way, most people accept idea that material from
Mars gets to Earth- what wrong the idea of some of the material not leaving Mars' orbit and evenually forming into moons.

It's not the odds on favorite but it's by no means impossible. Good thought, I'd not encountered it before.

It would be a smaller case of the same mechanisim that led to the formation of Earth's moon.

As for spectra and a difference between Phobos and Deimos, the proximity of Phobos to Mars' upper atmosphere-ionosphere would be a simple explanation for that.

Nobody, (not even E.P.!!) really knows for sure, yet.