Slime In Coolant Detroit DD16

[uncleal13]

I had a leaking hose clamp on my Webasto engine heater, I added a jug of Detroit Power Cool Plus OAT type coolant to the resevoir, the same type indicated by a sticker near the filler cap.

Over the course of about three weeks I noticed a dark line forming at the top of the coolant level with some strange slim bits that I couldn't identify. This past weekend it completely covered the top of the coolant.
It is a slime skim that feels like silicone gel in your hand, placed on a paper towel it leaves a yellow stain. I thought that it was fuel contamination due to this colour and the slime was a reaction between the fuel and OAT coolant.
The dealer this morning doesn't think it is fuel, just thinks it was the wrong kind of coolant. But I still had the bottle, and the Freightliner receipt confirming it was the right coolant.
Any thoughts what it might be?