It could have been any number of things considering in Tampa you have access to both Bay and open ocean macro's. Sargassum is a possibility at this time of year but not as common on the gulf coast.. more likely Hypnea, Gracillaria, Ulva, Enteromorpha and potentially Caulerpas.
I always advise a freshwater dip on macros before they go in the tank. At least a 30 min bath. Many macros will have predatory saltwater inverts attached, not so great in your tank.
Have fun!
>Sarah