PSCredential encrypts the string using that machines private key. The encrypted data has the same level of protection at rest as it does in motion. The level of encryption is dependent upon the certificate being used, so check your environment's certificate template for machine auto enrollment (if you use it). Alternatively you could just look at the certificate on the machine in question.
Having said that, the string can easily be decrypted with a powershell one liner (using .Net not the built in cmdlets). All someone needs is access to the system and the file. I strongly recommend you look at other alternatives, like a locally hosted password management tool.