That's a great looking G. Smithii! It even has the red inter-segmental bands that my old one had. I would feed a portion size no larger than your pinky fingernail around 2-3x per week. Frozen food of any kind (shrimp, clams, mussels, octopus, krill, whatever) enriched with Selcon is a great meal and it's easy to control portion size.
For live food, just put some small hermits or snails in the tank and watch him go to work. He'll most likely enjoy live brine shrimp too if you have access to those.
You can feed more or less than I've recommended, but that is just a baseline many people use. Just like with other marine livestock, if you feed too much and the mantis doesn't eat it, you'll begin to have water quality issues. Also, the more you feed, the more reclusive your mantis may become (fat, happy mantis shrimp don't need to forage as much)
Were you acclimating the mantis when you took these pictures? Mantis shrimp usually are not interested in food while acclimating because they are under a lot of stress. Best to release as soon as possible into the tank and let him hide for at least a day before trying to feed.
Yeah you need to let them go hide in the tank as soon as possible. Be prepared for the mantis to be pretty reclusive at first. Even my Peacock wouldn't come out of his burrow for around 3 weeks. I was really disappointed and thought he would always be that way. Then one day he just started coming out all the time. He loves to roam around now that he's comfortable with the tank.
Also, if you change your lighting sometimes the mantis may freak out a little bit. I changed to LEDs and my Peacock stayed in his burrow for about a week. He would peek out and look straight up towards the light then dart back into his burrow. He's completely comfortable with it now.
Yeah, great to hear that. at now, the new mantis alway hiding in his den, and sneak abit, and someone she will come out and back very fast. After chinese lunar new year, i will try to give her some shell meat.
For my old mantis, she alway hide in her tunnel, i got her for nearly 3 months, and molted a week ago. She look like not too interest with the frozen shrimps now... maybe try mix some for her later...
One more things, i think i am hard to get Selcon in Hong Kong, any other alternative? or some kind of Vitamin mixed drops?
Unfortunately I am not aware of an alternative to Selcon. You don't need it, but it does enrich the frozen food with key nutrients. Just make sure you don't feed the exact same thing everyday. I try to feed something different every meal. I'll let him kill a live snail or crab, then feed krill w/Selcon, then live brine shrimp, then clams w/Selcon, etc....