After washing المنی I feel like it does not clean completely. I feel some stickiness that is not gone fully until I add some soap in order to wash it completely. I wanted to know if this stickiness is Najis?
You must be sure that you have completely removed the semen مني . No Najis item can be made Taahir before removing Nijasah عين النجاسة
Stickiness usually means there are some remaining of the semen on your body, unless you applied sticky material like shampoo etc.
If you become sure that you have removed and washed away the semen, then it will enough for you.
Wassalam