Some questions regarding zooplankton

OK i have a few questions here....

Currently i have a 2ft about 7-month year old tank ..

Corals i'm having now is gonipora,mushrooms and buttons.

1) Do i need to feed them on zooplankton ? If needed, what i saw in the market currently have a few types.. one is liquid another is powder form .. which one i should go for ? anything i have to take note before purchasing the zooplankton ? it cost about $20 per bottle...

2) i feed my fish on prawn meat everyday once.. sometimes i squeeze the prawn into almost mashed form.. wondering will this be one of the food source for those corals ? in this case, do i still need zooplankton ?

3) the fries of the cleaner shrimps, is it a source of food for those corals i mentioned above too ?

4) my buttons are not opening for 2 days !! i wondering what happened...
water para: ammonia unknown (no test kits), NO2 <0.1 , NO3 5-10 ppm ... and i found that there's quite a number of "snail like" thingy crawling over the tank.. i suspect is they discard the button shell and come out ?


the snail-like thingy

please advise .. thanks