Towards the end of the JSSAT-14 webcast (around T+32:58), we see a thruster firing. Over the space of around 20 seconds, you can see it "icing up":
What is this that's forming? Is this a normal process for thrusters on spacecraft, or is it related to this being an upper stage that doesn't have much of a use beyond getting the payload to orbit - I would expect thrusters used in the longer term to not do this (especially since presumably anything that forms is not contributing to the actual thrust, reducing overally efficiency). What makes this different?