Hard to tell. A closer pic would help. It kind of looks like dying algae, but wouldn't you know it, if it was that?
Does it look like a golden snot, with air bubbles that increase through the day? If so it could be a new problem - dinoflagellates.
It looks more brown in the pic, so it may not be dinos. Regardless of what it is, manual removal, along with some hungry snails and/or a Scopas Tang should help. Most herbivore won't eat it when it's long like that. Manual removal gets it down to the length that they will.