Certified GKH Gardening Expert
This appears to be a spot from iron deficiency. This can be caused by, either, an actual lack of iron or too high of a pH.
You can add iron sulfate to bring the pH down, and use dolomitic lime to keep the pH more stable over a long period of time.
Here is an article that will give you more information on the plant: https://www.gardeningknowhow.com/ornamental/cacti-succulents/agave/growing-agave-plants-in-pots.htm