IRS Publication 969 (<a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p969.pdf">https://www.irs.gov/pub/irs-pdf/p969.pdf</a>) states:
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Note. Unlike HSAs or Archer MSAs which must be reported on Form 1040 or Form 1040NR, there are no reporting requirements for FSAs on your income tax return.
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There is a limit to how much an employee can be put aside tax-free into an FSA. For 2017 it was $2600. For 2018 it increases to $2650. Your employer handles this aspect. An employer can optionally choose to contribute more than that limit.
In the event that an employee chooses an FSA withholding rate that causes the limit to be exceeded and the employer makes a reasonable mistake in failing to catch it, the excess would then appear as W-2 income to the employee. (<a rel="nofollow" target="_blank" href="
https://www.irs.gov/irb/2012-26_IRB#NOT-2012-40">https://www.irs.gov/irb/2012-26_IRB#NOT-2012-40</a>)
