hitchiker id

I was feeding my fish then saw this slug thingy hanging out on the coralline on one of my acans. I took him out and have him sitting in some water but I’m confused on where it came from because I haven’t added anything new to the tank. Someone pls help me ID this thing. Is he friend or foe...

Not my crab but I'm curious. From the reef tank subreddit: Pics:

What kind of hitchhiker is this? Swims around by wiggling, its pretty cool ngl

I just cleaned a bunch of algae out of my sump and found these critters that look like tiny smooth (no hair or bristles that I can see) caterpillars. I’ve never seen these. In the photos they look flat, but they aren’t. They are nice and chunky. Any help on an ID is appreciated!

Hey guys, I got a shipment from reef cleaners yesterday and a bunch of my Astrea and turbo snails have these mollusks attached to their shells. They seem to camouflage with each snail’s shell. My guess is limpets. Can anyone confirm?

Hey all A while ago I noticed a tentacle coming out of the shady area of the live rock and haven’t seen it for ages. Today the tentacle is black and large and seems to be picking up left overs off the sand bed! Does anyone know what this could belong to? Sorry for the poor quality but I just...