Mystery stain on a thrifted shirt

We picked up this Van Heusen men's dress shirt at a thrift shop recently. The stain on the sleeve looked like curry sauce or something else turmeric-based, but washable. I treated it first (Melaleuca Pre-Spot), then washed it normally. The stain was less bright, but the fabric is still "stiff" in that spot. It hung dry, then I repeated the process with no improvement.

Any suggestions on getting it fainter? The thrift store provenance means I have no clue what it might be; y'all have all the information I have. (Care label included!)