The mò reading is the older, literal sense of the character: water rising up over something. It survives in a small set of mostly literary or formal compounds —
淹没 (to submerge),
沉没 (to sink),
出没 (to appear and disappear),
埋没 (to bury / stifle talent),
没落 (to decline) — plus the legal/administrative
没收 (to confiscate). In everyday speech the méi reading dominates; mò is what dictionaries and news write.