phosphorus deficiency?

Does not look like fungus. I think it is a fluctuation in PH or a phosphorus or calcium deficiency. But this is really minimal at this point and no need to worry by now. This can also happen when not watered properly. Dry spots in the soil can cause that issue, had that many times. As I don´t know if it´s okay to post links, i reccomend to google "grow weed easy", the first hit should be the right one. There you find many useful plant diagnosis relevant tips.

I'm pretty certain it's not a fungus. so you can rest assured there.

Do you know the pH in your soil? It's difficult to tell. Try to incorporate more pictures, especially the entire plant, as well as pix that show the stems. What nutrient line u r using and how little or often you feed and at what strength (ppm/EC) Environment numbers would be helpful as well.