This problem wasn't there when I got the car, but has slowly developed and now in this state. Although it isn't blocking the light but looks hideous to a person like me. You mean that I have to take out the headlights for a complete job right? and I'll be very thankful if you share the video 
I know it's strangge but it wasn't there the time I bought the car last year.
This problem is not there with my 9 years old Lancer but don't know why it happened to the Prius.
Sir Thanks a bunch for the detailed reply! Would Meguiar's headlight restoration kit do the job? Or if I use UC then which sealant would you recommend and how long will the sealant last?
Do you know anyone who can do this job well without messing up these expensive headlights?
It's there in the Headlight Restoration Kit, so should I go for it?
http://www.meguiars.com/en/automotive/products/g3000-heavy-duty-headlight-restoration-kit/