That small leaf is the oldest and has passed its "Use by date". Nothing to worry about there, old leaves die. The other ones may have developed that problem before you bought the plant. Those marks will stay there until the leaves die. Just check that any new leaves growing up look okay. Don't over water.

I find Alocasia/Colocasia difficult indoors - spider mites are a problem on them. I cut back on watering in winter where they often go dormant. If I see a leaf infested with spider mites I cut it off and dispose of. Alocasia requires less watering while Colocasia are often bog plants.

Outdoors they can thrive given the right conditions - lots of light, watering as needed, and a little fertilizer.

Check for spider mites. Mine went south really fast, but until I saw the webbing I had lost several leaves.