A321 Captain reported a fume event during arrival taxi. At arrival gate maintenance accurately diagnosed the issue.

Narrative

While in east hardstand after landing bleed was selected as pneumatic source before shutting down right engine. Shortly after selecting APU a crayon smell was noticed by me and I selected Auxiliary Power Unit off. We taxied to gate and shut down engine shortly before arriving at gate and deselected bleed and didn't notice the smell. Upon deplaning a deadheading Captain said he and his First Officer smelled dirty socks. We had shut down APU by then and were using ground air and power. Maintenance showed up and smelled the crayon smell and confirmed mid cabin dirty socks smell. The mechanic did a visual exterior inspection and notified me that oil was leaking from rear APU. Dispatch notified; report sent; cabin discrepancy form given to mechanic and we left with no affects or injuries. Great source of reliable information from the deadheading crew alerted me to a more serious issue than maybe what could have been construed as just crayon smell and maybe what I thought exterior smell from maybe hot brakes or otherwise. We gave APU well beyond 3 minutes before selecting bleed on.Suggestion: more awareness to speak up when a smell is not normal in cabin. Flight Attendant's in rear didn't smell anything or in the front but glad deadheading crew was able to articulate smell from their past experience.