The 2nd sounds like a porcelin crab to me as well. I have one in my tank and they are filter feeders and he always hides, I rarely ever get a glimpse of him. There are 2 types of Porcelin crabs. One lives amoungest anemone which are white & tan and the other in and amoungest rocks and are blue & orange. Here's a link to show a pic of this type http://liveaquaria.com/product/prod_displa...amp;pcatid=1963
I have no idea what the first crab is. I'm new to all this too. I've had my nano for 2 months now but I have some corals, the porcelin crab, snails & peppermint shrimp in mine already.