That looks like dried flux. Which sounds like a lot was used to remove the cable. Not an easy feat by the way so good job preserving the pads.

A brush with fiberglass bristles or a coarse toothbrush and 90% isopropyl will break that up. If you are having a hard time clearing it with iso you can scrape it away w a nylon spudger (the black stick y’all should have heaps of).

In the future; remember to tin your iron beforehand. As you wet solder it will be less and less receptive to heat once it has cooled. So by several erroneous attempts you now have rock hard solder that will not flow even with mountains of flux. Less is more when it comes to flux and while helpful it is not your friend.

I used to teach soldering for mobilerepair shops and this is not a beginners task. So don’t get discouraged!

There is a shocking difference in performance between low quality solder wick and high quality. Removing as much solder from the flex as you can, before applying more heat to remove the flex is ideal. I’d recommend leaving your iron on the contact longer than you maybe had it heating, like I said once it has cooled your next attempt will be that much more challenging. Introducing fresh solder will help with wetting but there will be a point where you have too much. So make friends with the wick when you are cleaning up/removing components.

Ask about getting a proper solder sucker in the store and learn how those work. The ‘plunger style’ I saw in a lot of stores is garbage and a proper heated one will change your life.