Owen,
Assuming the roof sealant at the caps and all the equipment is original, it is probably Dicor sealant. My experience has been not to attempt to add another coat of Dicor to the existing as it results in a very poor interface and actually forms a powder-like substance under the new seal, even though the seal looks normal from the outside. Best to remove the original seal if you are re-sealing. Not aware of any coating that would adhere well to both the fiberglass roof and the sealant.
Steve
