I want to use thermal paste for improving the thermal resistance between a semiconductor and a cooling element. But when I got it out of the tube, it was party transparent/fluid, and partly white/pasty. Can I nevertheless use it, or should I remove the fluid, or should I rather buy completely new paste?
A paste is often a collidal fluid, with particles and just enough liquid to keep them from sticking with each other.
If it separates a little, you will have the liquid and the solid part split.
Throw away the first mostly transparent part and use the rest as usual. You may find some bigger granules, if some solid regions expelled the liquid completely and sticked together.