What can cause a lump in the middle of the neck? (homework case study)
You ask for the fungus causing An erythematous , fluctuant, nontender mass (8cm x 6cm) was present in the midline of the neck.

On the brink of question closure I decided to do a quick search. Based on your descriptions, I found a possible culprit.

In the referenced paper they talk about a fungal Cryptococcosis infection causing, and I quote:


  [A] fluctuant, non-tender mass, five by five cm. in the right costal vertebral angle which was not red or warm. 


So this mass was found on the costal vertebral angle, but the remaining symptoms seem reminiscent of your case study. Fungi generally appear white when plated, as far as I know.
However, more information is likely necessary to confirm this genus and identify the species.


costal vertebral angle (CVA). Source: Pinterest

Reference
- Moore, J Invest Dermatol (1957); 28: 159–82