Do I need to perform ghusl janabah after watching something indecent and realise that I’m only slightly wet? Due to this confusion I don’t know whether I can pray or not without performing ghusl

by

You have to fear God and watch Him, and not commit this disobedience, for you are not safe that the Angel of Death will come to you while you are in this state.
With regard to your question, if you are certain that what comes out of you is semen, then you must do ghusl, but if you are not certain, ghusl is not required, and ablution is sufficient for you.