The name suction cup Caulerpa is used for a variety of species, but most commonly it is a form of Caulerpa racemosa. It is NOT illegal to possess or ship C. racemosa in (or out of) Pennsylvania, except to California. It is illegal to ship the feather Caulerpa taxifolia to ANY state.
There are state-specific bans in eight other states, I can provide you that info if you have a specific state in mind.