Is ghusl necessary upon waking from sleep with the presence of wetness and a feeling of contractions similar to a wet dream, where no water was seen, but a slight yellowish discharge was found, knowing that there is continuous yellowish discharge throughout the month? And is this wetness and dryness on the clothes evidence of previous semen or sweat?

What you saw is considered maniyy (seminal fluid), and ghusl (ritual bath) is obligatory due to it, whether you remember having a wet dream or not, because you found an unknown wetness that could be maniyy or something else.

The maniyy of a woman is thin and yellowish, and some of its characteristics may change at times. It is identified by two properties: its smell, which is like the smell of a man's maniyy, and the pleasure experienced upon its emission, followed by a cooling of desire.