Could be an Elysia (ornata ?). but pretty hard to say with these 2 photos only

Thanks Mathieu, You gave me a great clue. Unfortunately I have not any underwater photos from the species. Thanks again.

Some holoturoideo maybe!!!!

Dear Eelia, do you have photos of the rhinophores ? this could help. Mat

Dear Eelia,

I think it is possibly Plakobranchus ocellatus van Hasselt, 1824.

Dear Omer It is  Sacoglossan Sea slug....

Well Omar, Ravinesh was right, It is not sea cucumber, and the noted species by Ravinesh (P. ocellatus) is very similar to my collected species. Great Help friends, Thanks all.

What do the rhinophores look like? Is it associated with a particular algae?

Dear Nancy

I did not see anything adhesive to rhinophores, for example algea you said, The rhinophores are similar to Plakobranchus ocellatus, clear and uniform without any additional lobes and colored tips. I've found this kind of sea creatures are very clever to adapt their body color to  habitat where they live and this can change their morphological characteristics, at least their color patterns. Actually the information is scarce in our region and it made the identification more difficult. I decided to do a genetic identification to know what the species is.

where is it collected? from land, sea, estuary, and the depth? 

My reference to algae was with respect to habitat and what substrate you found it on. Sarcoglossans can have specific algae species association. 

Dear Baoquan Li

All samples were collected from about 10 meter depth of Persian Gulf