warranty on those should be 3 years.MSI Z370M mortar, I got it second hand off a mate who bought a brand new pc, so it's probably about 3 years old.
That doesn't look like capacitor juice--it's even on top of the release lever so came from above, plus those are solid caps. While a broken heatpipe would explain the high temperatures, those only contain a little water inside.
I vote foreign substance, with hair in it. If that many tiny caps had released all their goo it shouldn't even run.
it's possible that it may be some sort of oil leaking from the pump itself.
it's also possible that water leakage could've occurred and just turned color due to the surface it was cooking on but that should've dripped and/or run downwards towards the bottom of the board and not just pooled right on the area around the CPU.
unless your setup has the motherboard laying flat-out instead of upright.
Yeah it is quite possible that your AIO started leaking, and that is what's left of the coolant after the water evaporated. When the coolant ran out the pump motor burned out which is why the rpm of the pump motor went down to zero. You can certainly try cleaning it up with some isopropyl alcohol. You said the system still worked? You might just be lucky enough to clean that crap up, install a new cooler and be good to go.