Ya, I would be, that is a good sign it is aggressive and will probably eat your cleaner shrimp within the week. To me, that is way to big for a brittle star in a tank that size, but that's just me.. obviously it can be done because my neighbors tank is only 75G. Just keep an eye on it for now.. usually they only go after food and dead things like crabs/snails. If it is as big as you say, I would feed it at least 1/2 a cube of food a week so it doesn't go looking for food. If you put a piece of frozen food in your hand near it, the star will crawl right onto your hand and eat it. It is actually really cool and not as scary as it sounds. They don't bite.
