Guayas are a relative to the lychee and are also sometimes called Guaya Cubano,and/or Spanish Lime.
The Guaya fruit ripen during the summer and are found locally in small markets. Branches or bags of guayas are also sold by street vendors although rarely do you find guayas in the supermarkets.
The fruits are lime green and about 1 inch (2.5 cm) in diameter containing a single large cream colored roundish seed, which is sometimes roasted like a chestnut. The seed is surrounded by a yellowish/apricot translucent sweet juicy pulp. Juice from the pulp is also used for flavoring drinks.
To eat the guayas you tear the tough skin with a tooth or fingernail; the fleshy seed pops right out and into your mouth where you suck off the thin layer off pulp that surrounds it. The taste, to me, is sweet and fresh. The only drawback is that the juice will permanently stain clothing.
For more info on the growth and cultivation of guayas, or Melicoccus Bijugatus visit this site.