It sure looks like fungus to me too.
If you are lucky and it is ONLY on the rear element, it is an easy fix. Rotate the focus ring until the rear element protrudes as far as possible out the rear of the lens. The rear element assembly simply unscrews (counter-clockwise) by grabbing the black ring surrounding it. You should then be able to clean the fungus.
Here is a recommendation that suggests using hydrogen peroxide and ammonia as a removal/cleaning agent:
Ron Herron's "Mamiya 35mm Cameras" Fungus How-To
I would probably use some windex and hydrogen peroxide, and do a final wash with isopropanol (rubbing alcohol), with a blow-dry to remove residual fluids, and a final wipe of the lens surfaces with a very soft, clean, lint-free lens cloth.
I've cleaned the rear element of my 50mm f1.4 Super-Multi-Coated Takumar this way, but did not have a fungus problem, only some dirt and smudges.
When you get it fixed, post another picture so we can see the result.
-Joe-