cleaning a water bottle

Hello, the mouth peice to one of their water bottles looks black, or tarnished. My grandmother thinks it's just marks from them biting it, but it doesn't look like that to me. It doesn't look like mold either, but I'm definitely no expert. (Sorry if the pics are a little blurry!)

I recently switched my water bottles to the living world glass eco+ bottles. With my old plastic bottles I used a bottle brush and warm water to clean them which worked fine however I’m struggling to clean my glass ones with this method. Every time I clean them there’s still little particles...

So, one of my guinea pigs decided it'd be funny or a good idea to pee on their water bottle spout! Luckily, they have two water bottles so they're fine for now but I went to go wash it real hard, but noticed some actually got into the water bottle and not just on the spout. I tried putting...