Does a semen-like glistening inside the urethral opening upon waking up necessitate Ghusl, given that no dream was recalled?

We advise you to turn away from doubts and not pay attention to them, for their abundance can lead to obsessive whispers (waswasa). You are not required to perform ghusl (definition) unless you are absolutely certain that what you saw was the al-mani (sperm/seminal fluid) that necessitates ghusl. However, if it was merely an illusion or if it was possible for it to be something else, such as sweat, then ghusl is not obligatory for you; because the default state is purity, and this certainty is not removed by mere doubt.