If there is a glow on the dark portion of the Crescent Moon It is caused by sunlight that reflects off the Earth onto the Moon's dark side, you can see the Moon's outline, and it's dark features, even though only a thin crescent is bright. We see the Moon because of reflected sunlight. (the Moon does not generate its own light). At times, however, the dark part of the Moon glows this is EarthShine.
Just after the New/Dark Moon, the first Crescent Moon produces what is refered to as the "Old Moon" in the New Moon's arms. This occurs just after the New/Dark Moon just after Sunset. The same things happens to the old Cresent Moon in early morning.
Light from the Sun is reflected off of the Earth onto the Moon. Some of this light is then reflected off of the Moon back towards the Earth. Because of this we see part of the Moon illuminated by the Sun and the rest of the Moon dimly illuminated by this doubly reflected light. Earthshine is dimmer than moonlight because Earthshine is sunlight that has been reflected twice and moonlight is sunlight that has been reflected only once (off the surface of the moon). The reflectivity of the moon (its "albedo") is less than that of the Earth, which makes Earthshine even dimmer.
During a Crescent Moon, only a sliver of sunlight is reflected to Earth, But when the Moon is a crescent, the rest of it is illuminated by sunlight reflecting off the Earth, hitting the darkside of the Moon, and bouncing back to Earth. In fact, if you were on the Moon when it was a crescent, you'd see a full Earth!
We only see Earthshine during the Crescent phases of the Moon because during these phases only a small part of the Moon is lit up, The light of the Sun not only reflects off the face of the Moon, it also reflects off the Earth's surface. The reflected sunlight is referred to as Earthshine.