Well here's the thing with shipping live stuff... They will not cover it if something happens so the insurance is null and void. When I have shipped stuff I just told them it is corals and extremely fragile. They then stick the "Use this box as a football" stickers on them so they are treated better in the shipping process. I cross my fingers and send the corals on the journey of their life
Honestly though there is no reason to lie to them about it. Just be well aware that no matter what, they will NOT cover the package with insurance even if it was their fault. You take the risk of the shipment and can cross your fingers to see if it gets to the destination in one piece. I think they would treat the package the same whether you said it was live corals, canned fruit, or the family's best vintage moonshine. I have yet to be denied a shipment just because it is something alive...
If you are in a hurry sometimes it is easier just to say it's fish. No one questions fish, but I've had carriers ask about the coral. It's just basic curiosity they have, there is nothing wrong or illigal about shipping coral, but sometimes ya just don't want to answer questions.