I also agree ithat it does'nt look like aptasia as the "arms" are too think and short. Also in agreement it looks more like a zooanthid, but how will feeding it "tell for sure"? a zooanthid will also eat a meaty offering.
I always had a few hydroid jellyfish popping up in one of my tanks, they never got to o many of them though but i found they just disappeared as quickly as they came.
never experienced the combs in my tank though.
I have a mantis in my sump. He is really cool. This might shock a few people but he actually lives in there with a pistols shrimp and goby. I've had him for about 18 months now and he's never attacked anything much bigger than a cocepod rofl, but having said that i do make sure he gets a frozen...
firstly make sure it is without a doubt a fighting conch. Their are many types of conch that look ver simular and some grow very large. Then would it be possible to out it in quarantine for a month?
Stenoplax mariposa i'd say but i'm far from an expert try this link see what you think. its got some great info on them if nothing else.
http://home.inreach.com/burghart/