Please see attached photos. I think we have mustard algae, but not sure. We opened this year to clear water. Our CYA has been 60-70 all summer and FC has been between 4 and 7. All other test numbers are in acceptable ranges. Every now and then we get this line of some form of algae. Brush, vacuum, and it's back the next day. Always in the same places - on the ledge where shallow end starts to descend to the deep end, and a little bit in the deep end. It also likes the seams in our vinyl liner. If we run the FC up above 10 it goes away for a week or so, only to return again once FC is back to 4-6 range. Wife is reluctant to run FC up to recommended mustard algae shock level for fear of damaging the liner.
1) Do you think this is really mustard algae?
2) Is vinyl liner damage a valid concern if we stay within the recommended mustard algae shock level?