I wouldn't call them "active" (they mostly sit on the bottom or in the rockwork someplace, but they will definitely come out and greet the "food god". Be careful pairing them up, as I've seen larger, established males be a bit bullyish to smaller females until the girls get some size on them, and your newby is pretty small. We currently have a
I've never had Nassarius go after any of our smaller specimens. My first thought would be that they're starving (do you feed them?), or that you might have Ilyanassa obligata (mud snails/olive snails), which are predatory, and I've seen sold as Nassarius sp..
Do post some pix of your fish!
HTH