Date became resident if college student

It depends.  If you are only in Alabama to attend school, and are planning on returning to Florida when you finish school, your time in Alabama is considered a temporary absence.  Technically, you are still a Florida resident.  

The difference in the tax treatment is only on the state return, which requires you to file as a nonresident for any income you earn in Alabama as compared to a part-year resident return.  However, if you did truly move to Alabama, then you will use the date you moved into Alabama as the date you became a resident.

In either case, Florida has no state tax return, so none of your income would be taxable to Florida.  As mentioned previously, your Federal return is unaffected by this information.