Looks quite normal to me, silver particles plate out on the sides of plastic bottles and glass but it happens quicker and faster with plastics. Ironically it's benificial as it reduces the silver level in the fixer itself.
That's good to know. I, too, store my fix in plastic containers (reused Kodak chemical bottles from a lab). I use the two-bath fixer system and it was my first bottle of fixer where the bottle turned black. I figured it was the silver particles, but I thought it was a bad thing and time to replace the bottle. I still have the bottle (figured I'd clean it one day), and will put it back into service