identity crisis

while doing a bit of maintenance (daily turning over gravel, because MH
brings out algae within an hour of start-up) I noticed a small money spider
like crab.
this was only about 5-6mm in span. I found one dead and being blown about
and another hidden in the gravel. has anyone found anything similar I their
tanks and are they any help in the tank or bad.
I see a lot of tanks with only two small powerheads for circulation and the
tanks are clean, I use four and still have to rake over the gravel daily.
what am I doing wrong.

Robin