That's okay. I bought a used tank that had fragments of dried marijuana embedded in the sealant...lol. Secondhand adventures!
I don't know if you need to clean that paint out. Conservatively, I'd say yes simply because you don't know what kind of paint it is, and it could be toxic.
Taking the filter out and doing a 100% water change shouldn't destroy the cycle, as long as you keep the filter wet. My suggestion is to get a bucket of the old tank water, stick the filter in it, and run it as usual while you clean the tank. If it's an HOB filter, you might need to use a big square piece of tupperware, instead. You'll probably get a mini-cycle from disturbing the bacteria in the substrate and scraping the walls clean, but the filter is where the majority of the beneficial bacteria lives, and I'd expect everything else to steady out in around a week, maybe a little less.