This secretion is apparently normal for silicone rubber foam based thermal pads. The liquid is silicone oil, should do no harm to electronic components.
Don't like it either.
Maybe somebody can shed a light on this: is this common for 'cheap' thermal pads with low thermal conductivity or do higher quality pads also have this?
I have not received the thermalmod yet. Already bought Thermal Grizzly Kryonaut, but after seeing these pictures I might go for their thermal pads also. FujiPoly seems to be the best, but these are not so easy to get and rather expensive (at least where I am from).