Could you give us a photo of the whole plant?

Hi litterbuggy, I hope these are sharp enough:

Do the black spots and clumps wipe off, or are they in the leaves themselves? Are they on the bottom of the leaf too? Do they feel different than the surface of the rest of the leaf (raised bumps, dry, or musby? I'm really not sure what they are, but the information will be helpful to others who are more familiar with plant diseases.

The spots don't look like mite damage, especially because mites prefer tender new growth at the end of stems or branches. Check the backs of the leaves and that bud at the top of the tree for tiny dark red dots that easily wipe off or fall off if you shake or tap the leaf. They're really tiny, so a magnifying glass or loupe will make them easier to find if they're there.

The brown areas on the bottom leaves look like classic signs of overwatering or underwatering by the grower, so here are a few questions:

Does the pot have a drain hole? Is the soil very wet or just moist? What kind of soil is it--fine grained and peaty, or does it have larger grains that allow air to pass through the soil easily. Soil in the root zone can be saturated even when the top few inches feels dry, so stick a sharpened dowel, disposable chopstick, or a skewer all the way through the soil to the bottom of the pot to check the soil moisture.

Fine grained water retentive soils can stay saturated long enough to suffocate and kill roots, reducing the plants ability to take up enough water, which causes leaves the plant can no longer support to brown and eventually drop off, beginning with the lowest leaves on the plant.

The best way to deal with this is to repot the plant by removing all the old soil from the root system and replacing it with something fast draining and highly aerated, like Al's 5:1:1 mix made of five parts pine bark, one part peat moss, and one part perlite. However, there are ways to compensate for water retentive soils if you can't repot right away, including watering only when the plant needs it and tilting the pot 45 degrees after watering for 15-30 minutes to reason off most of the excess water from the lower part of the pot, as well as a few other tricks we can offer, depending on which way you want to go.

When I had spider mites, I found them by tapping/gently shaking the leaves, as suggested above. I held a piece of white paper underneath, then I stared at the little specks that fell out (they were like fine black pepper) until they started moving. Then it was war!!

Orherwise, could the small reddish dots be oedema? It's a physiological reaction to being overwatered at the time the leaf was forming, too much pressure ruptures some of the cells (something like that). Can't see well on my phones screen, but it's good if that's the case because it's not a disease or a pest, it can be prevented in future by watering better - see above!

Very cool camouflage pattern, BTW :)