Grass stains are tricky to remove, and since your trousers are white it will be most efficient to soak them in bleach. Rinse out the stain as best you can and then soak it in bleach. When the stain...
It will depend upon what the stains are, but CLR is probably the best first choice, especially for "hard water" type stains. It is an acid, albeit fairly mild; so any finish on the terracotta,...