In my old 55, I had 4 emerald crabs and saw them all the time. At least one was out in the open at any given time.
When I upgraded to my 240, I moved all the emeralds over. That was back in September. Never saw any of them ever again and thought they had been lost in the transition. I never replaced them either. Lo and behold, about two weeks ago I was looking in my tank with a flashlight in the night, and saw a massive, gargantuan emerald crab picking at the rocks. About 2 times the size of them when I put them in there. I went back the next night and saw another (different) one. Meaning, I know I have at least 2 emeralds still in there, with a gap of about 9 months in between sightings.
So no, it's not unusual for them to go missing, but I imagine they are much harder to "lose" in a 30 gallon tank than a 240.
I'd buy a couple more if I were you. They make good additions.
I too have a rock with that bristly turf algae you talked about. Is it the kind that is so hard and stiff, when I try to remove it by hand it doesn't even come off, as hard as I pull? And the only time I have been able to pull some off of the rock, it was so difficult that it removed part of the rock with it? If this is the same kind you're talking about, I have no idea what will get rid of that stuff.
