Coolant thermostat housing problem

Last night while checking my oil I noticed a dry pink crus on the rear of the thermostat housing on a connector. I have provided two pictures so maybe you could help me out. The first one is a shot of the left side of the engine. The leak is behind the cylindrical vacuum box. It is originating from the housing where a connector attaches. Is this the coolant temp sensor? Why would this be leaking? A bad o-ring? I have no engine faults yet. The second picture is a pic of the actual leak at the connector. Please get me some part numbers and some advice on how to change this oput properly. Thanks.

Is this the o-ring causing this to leak? On a 2002 golf do I order the blue or the green sensor?

Is that the thermostat? It looks like the vacuum pump. If you're sure it's the coolant sensor, just ask the vw parts guy for a sensor for your model/year car.

It's behind the vacuum pump in the first picture. It goes into the thermostat housing from the rear, towards the front of the car. It has an electrical connector at the end. My guages all work, it is just leaking a little bit and forming a crust.

Did you check for coolant migration??? Unplug the connector at the coolant reservoir to see if coolant is present in the connector and seeping up the wires. If so, you are in for the ride of your life in replacing wires! Otherwise, you might have a bad seal at the temp sensor. If you want to replace the sensor, you need a green one, part 059 919 501A. This thread gives you more info: Temp Sensor

Under the vacuum pump is the housing for the 3 COOLANT GLOW PLUGS that heat the coolant. These are the 12mm glow plugs everybody gets confused with the 10mm engine glow plugs. The coolant glow plug housing does resemble a thermostat housing. That's what I'm seeing here.

no coolant migration. I have the hole mod. It is leaking between the housing and coolant temp sensor.

It is the coolant temp sender behind the coolant glow plugs and vacuum pump. It is leaking between the housing and the sender itself. Any tips on changing this item without emptying all of my coolant? I am assuming it is the seal that went bad. I will change the sensor as well because I have the old black one.

There have been several recent threads on this. Here's one.

Along with above, I would say also to remove and then replace the reservoir cap to make sure pressure is released.