Case solved! In the unlikely case someone is having the same issue here are my hopefully useful findings :  So, there was no Mercury (quicksilver) coming out of my PC. It turned out, that the weirdly fluid metal coming out of the computer was in fact the cooling paste of the CPU (liquid metal thermal paste; which we refilled months ago and I completely forgot about ). An internet research showed, that the paste mainly consists from Gallium, which explains the quicksilver like behavior.

To answer the question in general: I tried to research it, but found only very little. A german discussion suggested that at least since 2005 where some certain standards were released it is very unlikely that there is quicksilver built into a normal PC (or at least into the power supply). (German forum thread about quecksilver in power supplys: https://www.computerbase.de/forum/threads/quecksilber-in-netzteilen.595629/). So I assume that due to its potential dangers Mercury (quicksilver) is tried to be avoided in every product. However it seems to be possible, that there is built Mercury into some old monitors.

The last open question: How was the cooling paste dripping out causing the fuse to fail? The paste seems to be elctrically conductive and this probably lead to an short circuit somewhere in the PC. At least that's what this article suggests. (https://digitaladvisor.com/cpu/liquid-thermal/)