Hello Meesupoon94
Your understanding is not enterily correct.
The Saver Day Pass is a day ticket for public transportation that allows you to use the public transport in Switzerland at will (hop on, hop of as many times and where you want) during its validity. There is no additional ticket needed, except for mountain excursions (cable cars, gondolas, lifts, etc.).
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
The Saver Day Pass is good for travel on the Glacier Express, there is nothing else you need.
The Saver Day Pass is a discounted pass (discounted = no refund, no amendments), yet still available at full and reduced fare. If you want to benefit from a reduced Saver Day Pass, as with any reduced ticket, you would need the Swiss Half Fare Card in addition.
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
Kind regards,
Roland