The only example of clouds at night i could find were in Shinobi 3. But even then the background is quite bright.
Oh you have many
more references available than that.
Few things to keep in mind with clouds:
- They are translucent. This means towards the edges of the form you start to get other tones and colours blending into them. This has numerous effects depending on the orientation of the lightsource. In cases where the cloud is somewhat between the viewer and the lightsource (in this case the moon), you get rims of lightness around the silhouette of the cloud. It also means you get much fewer hard edges, the form generally flows into itself very softly.
It also means as clouds get thinner/smaller, the tendency for light to completely pass through them increases, causing them to have a complete lack of shadow. If they are being hit by direct light, this will cause them to glow.
- They are in constant motion. This results in many tearing, stretching and compounding forms. The rectangular shape in your version is not typical.
- They are lit by reflected light from the surface of the planet. This, combined with the translucent nature of clouds, makes lighting them more complex. It depends upon how much light there is in the scene. In a modern setting, lightsources such as cities can light clouds up quite a bit. Reflected light of the moon on the ocean would probably give a bit of light onto the clouds as well, depending on how many clouds are in the sky to block the moon hitting the ocean.
Okay kind of rambling now.
Points are:
Your cloud needs to stretch/tear/compound on itself.
Too many hard edges
Shadow covers the entire underside of the form
Aside from that, it's grey. Try making it bluer, though I guess it depends on the scene you want to put the cloud in. What the light levels are, what the ambient light is like, how far away the cloud is from the viewer. These will determine the tinting of the cloud.
Study the references!