I noticed something like the 6th one in my tank yesterday, looked exactly the same. I thought it was the remnants of a snail as one of my hermits was right beside it. There was stringy stuff coming out of the shell so I thought the snail had been ripped out and left on the rock. If it's a chiton these looks pretty cool.
top one could be a spider or a nudi!
+1 on the chiton, mantis, small crab, and cucumber.
If u cant get a proper id on the crab i would remove it, safer than sry and most arent so good!