The can do ok in warmer temps, 78 would be the most Id put them in. Mid 70's is fine. Actually, the higher the temperature, the faster their metabolism, the faster they age out. A standard diffusa will live maybe 18 months. The sight had a great chart depicting it.
I used to QT my new snails too, but supposedly the don't carry diseases like other creatures do. It's not uncommon for an apple snail to retract and hide like that. They are nocturnal, and he might just be getting used to his new home. They need 7.6 PH to prevent shell erosion, less than 7 is too acidic. They usually need calcium rich foods. Calcium rich veggies are good for them to eat. I've got a cucumber to slice up and feed mine. He looks kinda green, algae covered shell?