Should I tell me dad?

So yesterday I was going upstairs to my dads room. When I leaned in I saw him with his laptop up and what appeared to be a video or watching a webcam and well his hand on his *cough* thing moving a little bit. When I said "dad" he pushed laptop up past his dingy and closed the screen. I tried to tell my mom but she said "he probably wasn't" But I don't believe her and well I am having nightmares about this please tell me what I should do and has any one else been in this situation