Does the slight white discharge that appears after waking up from sleep, sometimes accompanied by watching pornographic films or sexually arousing things in dreams, constitute semen necessitating ghusl?

To get rid of these matters, supplication is beneficial, and seeing what you see in your sleep does not harm you. You must perform ghusl (definition) if semen is discharged during sleep, and if what came out is madhiy, then you should clean yourself (istinja (definition)') and perform ablution (wudu') for prayer. If you doubt whether what came out is semen or madhiy, you may choose and assign it the ruling of whichever you wish.