White or clear stringy poo is usually a sign of internal parasites. You shouldn't have treated with ich medication. That'll kill your biofilter, which it probably already has, and put you in a far worse position than you started in.
If you really care about the betta you can try something like Levamisole. Honestly though, if the fish is this far gone, it's not worth it IMO
I'd let the fish pass away, add a few drops of bleach to the tank, let that run for a couple of days, then dump all the water out, pour fresh water in, lots of dechlorinator (on account of the bleach) and start over.
Either re-cycle the tank (which you're going to have to do after using ich meds anyways) or get some cycled filter media from somebody and stuff it in your filter between the cartridge and the filter wall.