We were recently on the Big Island of HI, and I got stung by similiar organisms. Our boat captain had a spray bottle of vinegar, spritz'd me, and 5 minutes later I could barely feel the stings. She said you can also pee on the stings (as they say for jellyfish), but I requested the vinegar.

This may not be the same creature, but maybe vinegar is a good treatment for Pica Pica also.