What would be the difference in a light ray that slows down (hypothetically)

and an object accelerating away from you?

Light must travel at the same speed in every context because photons have no mass. If those photons slowed to less than the speed of light, then their energy would be zero due to having no mass (do the math and see for yourself) and an object with zero energy is an object that cannot have any effect on the observable universe at all (since energy is the ability to affect things) and therefore you may as well say it does not exist.

An object accelerating away from you has mass, and therefore it can have different speeds, and it has different nonzero amounts of energy at different speeds. So there's a pretty big difference.

All the galaxies are red-shifted, because they're all receding. They are all moving away from one another at an incredible rate. If an object were to accelerate towards an observer, the light would be blue-shifted. It's due to the Doppler effect.

Light slows in a transparent medium such as glass or water. Light travel time is increased, but the frequencies (colors) remain the same.

If a source is moving away from you, the light is redshifted (the color shifts to the red).