Keep an eye on the mantis, and were gloves if you can. Be very careful, and if you get stabbed get into a hospital quick. Marine infections can be impossible to kill if they take hold, and you may have to amputate in a worst case scenario. I've only heard of that happening to a surgeon in australia however.
I don't know how small your nano is, but if its not at least 20 gallons it won't hold it too long. Are you sure its a peacock?
Feed it frozen marine flesh such as squid, shrimp, krill etc. etc. try to soak it in a food supplement like selcon once a week. Feed every once every 2-3 days and feed live foods such as snails and hermits at least once a month, preferably more.
Dan