What is the mysterious "CDC Serial" device and why do I needed it?
I've never found Windows to be very helpful in identifying drivers either. Here's some information for you on what USB CDC is. According to Wikipedia:


  USB communications device class (or USB CDC) is a composite Universal
  Serial Bus device class. The class may include more than one
  interface, such as a custom control interface, data interface, audio,
  or mass storage related interfaces.


Additionally, a resource called the USB Software Resource Center has this overview of CDC:


  There are three classes that make up the definition for communication
  devices: the Communication Device Class, Communication Interface Class
  and the Data Interface Class. The Communication Device Class is a
  device level definition and is used by the host to properly identify a
  communication device that may present several different types of
  interfaces.


All of that is developer-speak, but it really just means that a USB CDC driver helps your PC 1) understand what services your phone can provide it and 2) start talking to your phone to use those services. I don't know for sure what service the CDC Serial driver you see represents but I'm pretty confident that installing the official Samsung drivers for your phone would resolve the red X (the drivers that Windows installs automatically are sometimes generic and are often insufficient).

In its current state, with your phone connected as an MTP device, do you have any trouble transferring files or doing anything else you want to do with your phone from your PC? If you're not having trouble then it's possible the CDC Serial driver is only used for connecting your phone as a modem or serial device (as when communicating with your phone from the developer tool ADB). If you aren't planning to use your phone in this manner, you can safely ignore the failed installation of the CDC Serial driver.

I wouldn't stress too much over it and would install the Samsung drivers if you run into any trouble.