It can also be leaf hoppers, you may not see them but if their numbers are high enough they leave excrement.
Aphids, thrip, and spider mites you may be able to spot on the undersides of the leaves with a good loupe or hand lens.
fungal disease on mint usually shows up on the stems too especially at the bases as white fuzzy powder when it gets really bad. Fungal diseases usually show up in humid wet weather.
https://www.texasinvasives.org/pest_data ... ?symbol=37
Happy gardening in Hawaii. Gardens are where people grow.