What is going on here ??

[Snappy]

Hermit crabs, shrimp & some other inverts molt when their body outgrows it's present outer shell. The white critters are likely "pods", copepods, amphiphods, mysis shrimp, ect. That is a good sign that your tank is probably healthy. They are beneficial and something you want to have in your tank. Over the next several months you will see all kinds of things growing in the tank, bristle worms, feather dusters, peanut worms, ect. Take a flashlight and check it out after the lights have been out for a few hours and you will see a whole other world. Welcome to reef keeping.