That would be lye and indicates that you have a tiny gas leak there.
I’d love it if you’d send me a picture of it for my (eventually) trouble-shooting guide.
After the picture, remove the Tower Cap, clean the lye off (wipe first, then rinse several times with a damp cloth or sponge until the slippery feeling is gone) Don’t let water get down inside the machine (past the black collar).
Then seal the threads with Teflon Tape as per the Operation Manual and/or this video:
https://www.youtube.com/watch?v=sFXJaOSwwZs&t=8s
Put 9 to 12 clockwise wraps of Teflon Tape around the top of the silver water fill stem.
Put the Tower Cap back on, making sure it is straight up (not cross-threaded).
You shouldn’t need to remove the Tower Cap very often
