Try this( modified)
public class SimpleFormula : CodeActivity
{
[Category(“Input”)]
[RequiredArgument]
public InArgument<SecureString> Input { get; set; }
[Category("Output")]
public OutArgument<String> output { get; set; }
protected override void Execute(CodeActivityContext context)
{
var strSecPassword = Input.Get(context);
var strPassword = new System.Net.NetworkCredential(string.Empty, strSecPassword).Password;
output.Set(context, strPassword);
}
}