The white /gray film on your rocks is were something has died. Blow it off and siphon it out of the tank when you can. If the rock was removed from the water at your fish store and carried dry to your tank there will be some additional die off. If you carried it submersed in water then the rock...
THe super red mushrooms that I keep do not seem to get much bigger than 1 inch across. They are birght Neon pink when they start and darken to red, some times with white lines, as they age. They do not do well with bright (MH) lighting.
If you have hermit crabs, then go with the turbo snails. The margarita's size makes their shells excellent hermit homes and your hermits will kill them for the shells. The turbo snails are mostly just too big for the hermits to bother.