Though the "as would" construction here is grammatically correct, it logically implies that the movement of the pearls is a reality. However, the meaning of the sentence indicates that this movement is "just an image". Thus the simile should be a phrase introduced by the preposition "like" and not by a clause introduced by the conjunction "as". As mentioned, the "as would" construction would imply that the movement is a reality, something which isn't correct.
Awesome explanation, thanks for the follow-up Pooja. It DID sound weird as I wrote it, even though it worked grammatically. The weirdness was due to the meaning. Many times on the GMAT, I am thankful that I get to simply chose the answer with the less risky/suspect phrasing.