This is a character piece deeply influenced by the German Romantic lied tradition. It is distinctive for its lush harmonic language and contemplative, nocturnal atmosphere.
The main challenge is achieving a seamless legato and balancing the inner voices against the primary melodic line.
Comparable difficulty and character — good candidates for what to learn next.
'Mondnacht' (Op. 5) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 'Mondnacht' (Op. 5) lasts about 5 minutes.
'Mondnacht' (Op. 5) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Gymnopédie No. 1 by Erik Satie, Autumn Song by Cyril Scott, Andante by Raymond Moulaert — see the full similar-pieces list on this page.