That is pretty cool.
Lysmata spp are not known to be easily raised. They will readily spawn, as they are hermaphroditic, but their larvae have a long development period of 40+ days so this makes them difficult to raise effectively and economically.
Here's a short piece on it:
http://reefkeeping.com/issues/2002-10/nftt/index.php
Your seahorse tank must have an ongoing population of foods for peppermint shrimp larvae and the right conditions (low flow). This is indeed very rare to have shrimp unexpectedly reproduce and survive to adult stage.