I would say it hitchhiked in from a new rock or coral that was placed in your tank.
For reproduction I went directly to this article written by Dr. Ron....
http://reefkeeping.com/issues/2003-11/rs/index.php
It says:
"Sea urchin sexes are generally indistinguishable by the casual observer, although some researchers have learned to "read" subtle differences in shape that allows them to predict what gender the animal that they are examining is. They are broadcast spawners, liberating their gametes into the water column, where eggs and sperm meet and form a new individual. The embryonic sea urchin develops in the water, where it becomes a feeding, swimming, larva within a few hours to a few days, depending on the species. The larvae may live a long time in the water, and they can become quite complex and relatively large animals, up to a few millimeters across in some cases. If conditions are optimal, these larvae may actually bud off little clonal tissue blebs that develop into more larvae as well. Eventually, the larvae will settle to the substrate, and metamorphose into a juvenile sea urchin. Once they settle out of the plankton, sea urchins may live for a long time. Their ages can be determined by counting rings in some of the ossicles, and average ages in some populations are on the order of twenty or so years, with many animals living quite a bit longer. As with all echinoderms, they don't die of old age, but succumb to disease, accident or predation. They play the odds of life, and eventually they lose the bet."
Hope this helps,
Kaye
