It seems like a case of territorial aggression. It is common for clownfish to turn against each other when settled except when they are a pair. You might get away with aggression if your tank is big enough.
Cheers
They sure look very similar to those we used to have at our shop except for the color of their shell.
http://www.m3morph.com/main/products-page/in-stocks/invert/nassarius-snail/
Cheers