Nocturnes typically invoke the imagery of the night, nocturnes are also typically single character pieces with no overlapping theme. Clair de Lune is actually not a single piece but actually only one part of the Suite bergamasque. Why this suite is almost never played in its completeness is also something that is lost to the ages. It is also worth noting that Debussy was actually not all that happy with the publication of this suite as it was composed before his style matured.
Also, the style of the two is markedly different. The Nocturne style was a decidedly romantic era composition, and Chopin is one of its main proponents did nocturnes in a style that was wholly alien to Debussy.
Debussy had a great reimagining of the harmony that the romantic era simply never had. Debussy wanted his music to invoke the imagery that a Monet painting did and a lot of his music was with this theme in mind.
Interesting enough Debussy did write Nocturnes although not specifically for piano.
Debussy wrote an "introductory note" to Nocturnes as follows:
The title Nocturnes is to be interpreted here in a general and, more
particularly, in a decorative sense. Therefore, it is not meant to
designate the usual form of the Nocturne, but rather all the various
impressions and the special effects of light that the word suggests.
'Nuages' renders the immutable aspect of the sky and the slow, solemn
motion of the clouds, fading away in grey tones lightly tinged with
white. 'Fêtes' gives us the vibrating, dancing rhythm of the
atmosphere with sudden flashes of light. There is also the episode of
the procession (a dazzling fantastic vision), which passes through the
festive scene and becomes merged in it. But the background remains
resistantly the same: the festival with its blending of music and
luminous dust participating in the cosmic rhythm. 'Sirènes' depicts
the sea and its countless rhythms and presently, amongst the waves
silvered by the moonlight, is heard the mysterious song of the Sirens
as they laugh and pass on.