it looks like you've already got some snails. I would recommend expanding your cleanup crew gradually, so that you avoid overstocking and starving them (like you mentioned when the food source runs out). more than once I've bought a bundle of 3 or 4 algea eating snails at once (to get a group discount) for my 30 gallon, and had them dieing off after a couple weeks after they cleaned off the rocks (i don't have super lights, so no super algea growth).

Off the top of my head, some good food/detris scavengers are your nassarius snails, brittle/serpent stars. Algea eaters are turbo, astrea, nerite snails, hermit and emerald crabs. Lots of other options as well. I recommend getting a small number of several types, instead of a large number of only 1 type; because each can have different types of algea they prefer.

The white things are probably small feather dusters, which are fine. They can free swim for a bit when young, then pant on a surface not to move again. the sponges usually require low light to thrive, so over time if you don't mess with the live rock, they'll probably show up in the shaded spots.

anyway, that's my 2 cents.