Phobos is closer to it's planet than any other moon in the solar system. It's less than six thousand km above the surface of Mars. Phobos is moving very close to Mars, and there is a possibilty that one day it'll crash into Mars. Phobos is also one of the smallest moons in the solar system.
Phobos rises in the west, and moves very quickly across the sky in the east. It is so close to Mars' surface that it can't be seen above the horizon from everywhere on the surface of Mars. Phobos could be a an asteroid captured by a nearby asteroid belt.
The moon Phobos may be made out of carbon-rich rock like C-type asteroids. But it's density is too low, so it can't be natural rock. It's most likely made out of a mixture of rock an ice. The moon Phobos also has many craters on it's surface.