Looks like a scud to me. Some folks say harmless but my experience has been different. I had them in 10 gal. shrimp tank. They multiplied to the point that all I saw swimming in the tank were scuds. They became very aggressive and I saw them attacking and eating shrimp. They also destroyed my Christmas moss. Put some rasboras in there and they helped reduce the population but also feasted on baby shrimp. So I ended up breaking down the tank and starting over. Now scudless and happy. They originally came in on plants from one of the LFS in my area. Bottom line: who needs them? I'd do away with the little pest.
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