The main ingredient in linoleum floors is linseed oil. When you lay something on the floor, be it a rug, a decorative pot, or a bookcase, you will see yellowing when the object is removed. This is the linseed oil rising to the top.
The solution for this type of discoloration is to expose it to natural light. Open the curtains, and allow as much natural light as possible in the area. This will dissipate the yellowing but will take some time, perhaps days, or even weeks. Do not use bleach or any other strong cleaner on the area. It will just make it worse.