First of all, you can not mine just one coin, you mine x11.
Also, Ninja is not a specific coin. It can be any coin, they call them ninja so that others wont know what they are and the difficulty stays low.
Here is what the FAQ says:

"Ninja coins are coins that aren't found on most other pools, and to help secure the profitability, they are set as Ninja. Also coins that are already severely behind on payouts have also been set as Ninja. You will get rewards for finding these blocks."