Definitely a porcelain crab, which is actually a type of lobster if memory serves me correct. They come in a variety of colors and will mostly hide during the day. They have 2 big fans that they will use to filter feed out of the water column and are my vote for coolest hitch hiker. Non harmful to corals and can be kept in small groups. You're lucky!
I was actually lucky enough to get 3 of them. I also received 2 of what looks like a starfish. And of course thank you for all your help. This forum is the greatest. I don't post much, it is a lack of knowlege thing again thank you so much.i