Basically many people think that Scald is hot water, so it's type should be Water + Fire (Same as Flying Press). But it isn't. It is just hot water, not fire. You can burn a stick (wood) with a fire, but not hot water. Pour hot water on it, you will find the main difference. When you pour hot water onto the grass (or a plant), it surely absorbs it the same way it absorbs normal water. Similarly, pour hot water on Ice, Steel or a tiny bug. Ice does not melt at the same rate when it is exposed to fire. Steel and Bug doesn't burn easily when hot water is poured, but it takes just a few seconds to burn them by Fire. So don't think Scald is literally Water + Fire type. It is just Hot Water. So it is not super effective against Grass types.
Hope this helps :)