Good morning, serenity
I'm just going to take a wild guess that the blob at the bottom of your pool may be floc congealed with dirt and debris...some real estate agent or freddie mac maintenance guy may have thrown it in at some stage of foreclosed life...have you tried to vacuum it to waste, or does it clog? Or can you try to bring some up with a good leaf rake?
If it were the vinyl, you'd have to change the liner anyway...and changing the liner is always a relatively possible scenario with a foreclosed pool....sitting in acidic conditions for so long really can degrade an aging liner.
I was lucky and have gotten another 5 years outta mine, which still isn't leaking, (but by now, at age 15, has worn a lot on the bottom from the robot) but in my case I'd have changed it by now were if not for the extra expense and hassle of dewatering (high water table) and then another grand for two trucks of water (am on well).
I will send you the mojo that you have the same luck...
Either way, i highly doubt the fibrous goo you're encountering is the liner itself. Another possibility causing the "white" appearance would be if the print on the bottom of the liner has faded out. For roughness, its also possible the vinyl has wrinkled from acidic conditions.
With respect to the following conversation:
I have found that using the upper "mustard" shock value as the high limit (eg dosing below by a point or two, but above regular) had no negative effect in swamp recovery...and definitely would not consider dropping BELOW regular slam target remotely ideal as that's how IMHO many stall or prolong slams for weeks.
In warm-enough water with algae, it will simply outpace the slam if the "kill zone" is not maintained. That's why color change is a helpful guide...if the color is not changing at least slightly, daily, you are not gaining ground. Sometimes that's because the cya level is higher than read, and sometimes its because if you dose to 12 and its below 12 in the first hour and you're away for four hours, you've spent 3/4 of your "slam" not "slamming"
But that is not to say that maintaining slam/shock value AT mustard algae rate for prolonged periods is ideal either...the mustard algae slam level is meant as short-course elevation. You can determine the mustard shock value by entering your numbers into pool math calculator.
Hope that helps clarify.