I had some bad advice from an LFS and got a clam when my tank was still in cycle mode, the clam didn't make it. I no longer deal with that LFS but learned a lot about clams after that incident. If you confirm it is a clam then see if a fellow reefer can help you or maybe your LFS to keep the clam until your tank has been up and running for at least 6 months. It's not just about completing the cycle, you need a mature system. You may get lucky but it's really not likely.