From the Hotline: Enrollment in FSA and HSA Accounts
Question: Can an employee enroll in FSA and still have an HSA account, just not contribute to it?
Answer: Once an employee is eligible for and establishes an HSA, the funds remain his/her property and s/he can use those funds tax free as long as they are used only for qualified medical expenses. According to IRS rules, if the employee then takes part in a health FSA that is not a limited purpose FSA or post deductible FSA, s/he can no longer contribute to the HSA, but s/he can continue to use funds in the HSA as long as s/he is not using funds from the FSA for the same expenses.
For more information, review IRS Publication 969 available at http://www.irs.gov/publications/p969/ar02.html#en_US_2013_publink1000204025.