Inverter coolant milky

Like what? A picture might help.

Either way, I can't see how anything besides dumping the wrong stuff in the tank would contaminate inverter coolant.

 

That's looks like an oil-coolant emulsion. I can't think of any reason for any type of oil based substance to get in the inverter cooling system unless someone added it. (Doesn't look like stop-leak or anything). Too much oil in a cooling system can attack and degrade rubber hoses and water pump seals.

Even if there was some kind of one in a million cracked transmission case, I wouldn't expect trans oil to get pushed into the coolant- rather the other way around. (Might want to check the trans oil level and condition anyway.)

It can be very difficult to clean oil out of a cooling system. Lots of water flushing, some gentle degreasers (Simple Green?), more flushing. Then refill with fresh Toyota pink coolant and bleed.

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maybe someone topped up with their milkshake in an emergency

 

I don't have a diagram handy, but doesn't the inverter coolant loop also run through the transaxle case? Not sure that is the right term, the place where the two electric motors live. That is filled with an oily substance. If that was leaking into the coolant it might produce that milky white. More to the point, if the coolant was in turn leaking into the transaxle case that might be bad for the components located within. Especially if it chews up insulation, or causes spots to overheat.

Diagram here, that component underneath the inverter where a hose goes in and another comes out:

Inverter coolant and engine coolant flowing through the same radiator? | PriusChat

 

Yes I've seen it, just never in a Prius. Usually it's the result of a leaking oil or transmission cooler.

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