Neither, AsnEncodedData or any tool will decode unknown extension. Each extension has specific syntax and common extension decoders are implemented in Windows. For custom extension you have to write your own decoder based on extension's ASN module. I suspect that your extension is a simple IA5String, so you can decode this string:
[PKI.ASN.ASN1]::DecodeIA5String($cert.Extensions[8].RawData)