If it's powdery and yellow or mustard color, then it's likely to be yellow/mustard algae and that takes more chlorine to kill and to keep away. I would shock at a higher level 20-25 ppm FC and you can lower your pH some (say, to 7.2) before adding the chlorine since the large amount of chlorine will raise your pH (which makes it a little less effective). Keep the chlorine up until the powder is gone and does not reappear and keep brushing it so it mixes with the water and gets full exposure to the chlorine.

You probably had your CYA too high at 60 since that would mean needing to keep your FC at a minimum of about 7 ppm FC. If you maintain 30-40 ppm CYA, then you could keep a minimum of 5 ppm FC (maybe a little less) and probably keep this mustard/yellow algae away. Some users report that they need a higher chlorine level to keep the algae away -- roughly the Max level on Ben's chart or the 0.07 column in my chart while others are OK with the mid-levels or the 0.05 column.

Do you remember what your FC level was when this algae first appeared? That will help us confirm or adjust our recommendations.

Richard