Is the fluid discharged due to arousal seminal (manī), pre-seminal (wadī), or something else? And does it invalidate the Ramadan fast and necessitate ghusl?

The predominant fluid that exits upon seeing something that arouses desire is called madhy (pre-seminal fluid). It is a clear, viscous, impure fluid that does not necessitate a ritual bath (ghusl), and its emission does not invalidate the fast.