I have a BS and certification in elementary (K-6) ed, am currently teaching middle school and pursuing a master's in reading, but would like to know what is involved in becoming an ESL teacher.
I am semi-fluent in Spanish, but is that actually necessary?

First you have to domain another language (Spanish preferred) (written /spoken) then search in the State School Board for any job openings and apply.

I couldn't find licensure information. Try the link below. It is the CT state site for licensure. It's linked on the CT state Department of Education website (also below).

Check with your district Human Resources person. He or she may have further information. Good luck!