Without further info, best guess is spider mites.

Doubt that's the reason for the brown spots.

Beyond that, there's a passel of reasons why container-grown plants havae problems -- among them compacted potting mix; root bound; accumulated fertilizer "salts" which are not necessarily sodium salts; poor drainage, or not drainage; poor watering practices; and more.

So, as was suggested, too little info to offer a viable guess.

Unfortunately, I have no additional info. It just happened recently. The tree was fine before that. I planted it 9 months ago, so should not be root bound yet, using FP very moderately, drainage is fine based on what I see after watering (Kellogg's citrus mix has a very good drainage IMO). We had some rainy days and fogs and soil was wet (not soggy) for two weeks, so maybe that. I just hope the tree is not sick.

As you are in the Bay Area with high humidity, fog and rain, it could well be Alternaria Citri, (Mandarin Brown Spot) which is triggered by rain events, fog, or sudden changes in relative humidly. The actual condia are airborne and carried by wind. Your tree currently looks to have a low level of infection. Copper fungicide sprays are used for control. Treat it now and it should be OK.

Will this work?

That will work,,, It is not alternaria; but could be another simpler fungal infection.

Thank you, guys!