I've been told by more than one local dive masters that pica pica is some form of plankton that inhabits the water from around April till June.
It seems to be in certain areas....so not everytime you go diving will you get it.
Any exposed body part is fair game for pica pica.
They suggest to take hot water...as hot as you can stand it....and a wash cloth and hold the cloth over the bites. This forces the critter out of your skin.
Using Windex on it...(it's the ammonia in it)....will also work.
Easier said than done....Don't Scratch!
Try some Hydrocortisone cream or Benadryl cream for the itch.
I have been fortunate so far....never been bit.
Hope this helps...let me know if you try one of these remedies and they work.