In the Name of Allah, the Inspirer of Truth.
Assalamu Alaikum Warahmatullahi Wabarakatuh

Ghusl does not become fardh in this case. What you have experienced is preseminal discharge however Wadhu is broken. Ghusl becomes fardh when the semen is ejaculated with desire (shahwah). However, this is not the only instance where Ghusl becomes necessary but for the scope of this question, this is sufficient.

And Allah knows Best
Wa Alaykumussalaam Wa Rahmatullahi Wa Barakatuh