Noone knows how long they have been around, perhaps from the dawn of time! 
My guess is that they were intoduced at least a year ago because that is the last time Rick Bangerter added anything to his tank before he left for Brazil. These seem to be detritivores since there seem to be no missing coral.
As for your last question, there could be many. My guess is that there are hundreds or thousands of babies.
Frankly, I wish there were more people interested in discussing these ugly things. If there were some interest, I thought I would bring them to the meeting tomorrow but alas...
Mark