I learned that when I exchanged my Nissan Diesel with newer one that was meticulously cared for. The older engine cooling passages had the gel like white near solid stuff and I think I recall some corrosion. The newer maintained engine was clean and bright in all of the coolant circuit. Now I change the coolant every 3 years roughly(this reminds me, it's due again).

Other than that, I've had problems in the that area with the radiator cap not sealing correctly. That required sanding the surface that the rubber washer seals against, until the seal was tight. You may have to get a new cap and clean the top surface as that is a secondary seal of sorts(when the pressure/temperature seal opens). Then if the passage is clean(the one you show plugged) and the weep hole from it, the hose not clogged to the expansion tank, you should be all set.

Changing coolant and maybe flushing/cleaning could solve some of the problem as well. Hope this helps.