Is this language semi-decidable?
You can start by considering this related problem:

$$L'=\{w\#x\#n\,|\,\text{all states are reached when running }M_w\text{ on }x\text{ within $|n|$ steps}\}.$$

Argue that $L'$ is decidable. Then, noting that

$$L=\{w\#x\,|\,\exists n.\ w\#x\#n \in L'\}.$$

should give you an idea about how to proceed.