Well if it is a red macro, I haven't kept them so I have no personal experience but from the short reading that I have done they seem to not be predatory, but they are opportunistic nippers of everything they can get their claws on.
The little guy seems very contented with his situation. He eats anything he sees and likes to roam around the tank. He doesn't follow my finger around, but he definitely knows I'm there. He'll come over and watch me watching him.
And he's definitely a Red Macro. I've been reading up and it seems they are a good beginner shrimp. Which translates to him not dying on me. Yay! And I like the guy's behavior. I cringe when I have to put my hand in the tank. Exoskeletal creatures make my skin crawl a bit. But I do like him.
I blame the stores for not carrying enough freshwater invertebrates, every time I visit one there are the amano shrimp, glass shrimp, dull looking brown cray and mystery snails and rarely anything else.
I need to start buying online but I am weary of buying stock that I can't see before hand.
It's probably a Macrobrachium rosenbergii (well, M. dacqueti due to taxonomic revision ... pending an appeal to the ICZN for reassignment back to M. rosenbergii). They're pretty interesting in the aquarium, but they get a lot bigger than most people suspect.
This is what I mean by big:
To be honest, though, that kind of bulk is pretty exceptional ... and other (much smaller) species of Macrobrachium are occasionally also sold as "blue prawns." I could try to confirm ID if you post a photo.