Thermal fuse? heats up and separates if it gets too hot (too much current)?

Looks like a Thermal fuse to me as well

I was also thinking fuse. No magnets or coils close by. Measures open circuit so that's a problem.

FIFO:

 

I was also thinking fuse. No magnets or coils close by. Measures open circuit so that's a problem.

 

Could be open by design and close when the enclosure gets too hot and shuts the pump down nicely or reduce flow/load?

Reed switch normally has glass around it

interesting, it does close with a magnet

https://www.jaycar.co.nz/miniature-glass-reed-switch/p/SM1002

think this is it, still can't figure what its purpose is on the board

OK, figured it out. The controller card is mounted next to a water tight pressure vessel. There will be magnets inside there that drives the switches.

Then it's a Reed switch

Linux:

 

Reed switch normally has glass around it