Is this language semidecidable?
If $L_1$ is semidecidable, there's a TM $M_1$ that takes a string $x$ and always reliably halts and answers "yes" if $x\in L_1$. If $L_2$ is decidable, there's a TM $M_2$ that always halts and reliably answers "yes" if $x\in L_2$ and "no" if $x\notin L_2$.

Make a new machine $M$  that when given an input string $x$ first runs $M_2(x)$. If the answer is "yes" then you know $x\notin L_1\setminus L_2$ so have $M$ halt and answer "no". If, on the other hand, $M_2(x) = "no"$ then $x$ may or may not be in $L_1\setminus L_2$, so then run $M_1(x)$ and do whatever $M_1$ does. It may never halt or it may answer "no", but if $x\in L_1\setminus L_2$ your $M$ will correctly answer "yes", proving that $L_1\setminus L_2$ is indeed semidecidable.