Is ghusl required upon seeing a light, dry white fluid on the underwear after waking up, with no recollection of a wet dream or feeling anything being discharged during sleep, and being certain that it is not a wet dream?

If you wake up and find semen on your garment, you must perform ghusl (ritual bath), even if you do not recall having a wet dream. If you are uncertain whether it is semen (maniyy) or pre-seminal fluid (madhiy), you have a choice: if you wish, you may consider it semen and perform ghusl; and if you wish, you may consider it pre-seminal fluid and wash what it has stained.