Is pouring washing powder on the penis to remove slipperiness after ghusl from semen considered excessive, and can water discharge be accompanied by the presence of slipperiness?

What you mentioned is reprehensible and stems from waswasa (whispering of Satan/obsessive thoughts), so beware of it and do not indulge in it. Wet semen is removed by water and does not require soap; it is sufficient for you to pour water on the affected area until you are reasonably sure that the impurity has been removed, then wash the rest of your body. Do not pay attention to the illusion of stickiness remaining, and know that semen is pure, so even if some of it remains, your prayer is valid.