Please help, sick betta with mystery illness

Yesterday I noticed my betta Pablo in his little cave and not as active as usual. I did my normal vacuum and water change. Today, he's at the top corner with what looks like a stringy substance hanging from his back fins. I also see what could possible be a parasite from his anus.

Husbandry spec: 3 gallon, well cycled tank, heated and filtered, planted. I test with strip but according to them, everything looks good.