The pass only covers MTR subway trains. Not for ferries, trams and most buses. That's why you still need and want an Octopus, whether you get that $55 pass or not.
As for train costs, without crossing the harbor, it's around $5-7. Crossing it and you starts at around $9-10. Up to $20+ going to parts of New Territories and to Disney/Tung Chung.