Gaz pretty much described what Pica Pica is, and it is called different names in different parts of the world, sea bather's eruption, sea scratch, etc.
There is excellent information provided by Green Reef, a non-profit reef conservation organization in San Pedro. You can visit their website and read some of their articles on Pica Pica.
There are many other good articles on marine life in the section called Reef Notes and I am sure they would appreciate the support of some of you 'regulars'. //ambergriscaye.com/greenreef/briefs62.html

Couple of points:
- Sea grasses have attached to them thousands of minute living plants and animals, one in particular, a tiny anemone (animal with tentacles) that can deliver a rather nasty sting.
- Not all jellyfish sting, sometimes the larva (baby) delivers a worse sting than the adult as in the thimble jellies
- If an elongated welt like wound occurs on your body after swimming, you may possibly have come in contact with the trailing tentacle of a larger jellyfish such as the box jelly or worse yet a Portuguese man-o-war (not a true jelly. Pica Pica will not deliver this type of sting, it is more like hundreds of little pricks with a needle.
-Finally, don't let this deter you from going in the water - it hasn't me and I had the worse bout with Pica Pica a few years back off of one of the southern cayes.