I saw in one of your videos that if a dark layer forms on your turbah after a while, then it is not permissible to pray on it.
1. I didn’t know about this ruling so does this mean my previous prayers are invalid?
2. I cleaned it underwater with my hands but the layer still remains. Is this ok or does this mean that I need to scrape it harder using some tools?

1. That is fine and you don’t need to repeat
2. You need to remove the layer or use a new turba