I was looking in the engine bay this morning and found this. It's not the water pump, but the elbow behind the water pump. It looks like it had leaked before sometime in the past, but not sure if still something I should be worried about. Should I wait until it gets to be a bigger issue/leak or fix it now? And is it the thermostat housing? Thanks in advance.
That's the water inlet elbow, callout 16031 below. It includes the engine thermostat. 2012 Toyota Prius Water inlet sub-assembly, with thermostat. Side, engine, radiator - 1603137010 - Genuine Toyota Part
Not quite sure what you're describing, but the black dot indicates non-reusable, wherever it is. It's only explained (with text) once. These "non-reusable" notes can be taken with a grain of salt though, in some places more than others. If it's something critical, or in a place requiring a lot of effort to revisit, it's maybe best to heed them.
