疒 (illness radical) + 志 (mark) gives both the meaning, a skin mark, and the sound.
culture
In Chinese face-reading, moles in different positions carry different fortunes. A mole near the mouth is considered auspicious - a 食禄痣 or 'food-fortune mole'.
Sickness radical wrapping the top-left, drawn as a person lying on a tilted bed. It marks 痣 as belonging to the medical/skin family - the same wrapper appears in 病, 疼, 痛. Here it frames a minor skin condition rather than a serious illness.
Right-bottom 志 gives the sound exactly - same syllable, same tone. It also lends meaning: 志 carries the sense 'to mark, to record', and a 痣 is a mark the body records on the skin from birth.
