Difference between Hashcash's Proof of Employment and Ethash Proof of Employment Proofs?

Hashcash and Ethash are two different things and not directly comparable. Hashcash is the idea of ​​using hashes for a proof-of-work system. it can be used with any hash function.

Ethash is a specific hash function. It is used in a hash-hash system, especially in Ethereum.

Hashcash is not comparable to Ethash because it is not a hash function at all.