In total I have 5 mimosa trees between 5' and 8' tall. All of them are pretty much in identical living conditions. Same soil, same location, sun, watering schedule, etc. This one is the only one with these symptoms. I first suspected that I was over-watering so I reduced watering it about 3 weeks ago and drilled some extra holes in the pot (bucket) that it's growing in to improve drainage. It's still growing but even the new growth is pale. Since it hasn't improved any, I don't think it was a water issue after all. I'm thinking it must be some sort of nutrient deficiency now.. I will test the soil tonight.. maybe that'll give me a clue.
I found mealybugs on my largest tree yesterday but so far that appears to be isolated to just that tree. No sign of them on the pale tree. So I don't think there's any correlation.
If your mimosas are in identical environment, i.e. they are in the same temperature (cold weather can make that happen in the case of mimosas), they grow in the same type of soil (with enough drainage, not too alkaline) and they receive the same amount of light and water, and only this one looks like this, I'd say you are left with two options: a nutrient problem, or a pest problem.
The pest is unlikely because it usually leaves other visible symptoms, and nematodes will hardly do that to an adult plant... So I'd try with nutrients. If you can measure the EC to be accurate with the concentration you give them, awesome. If not, feed her a conservative amount of good NPK fertilizer with a high N ratio (say 4:1:1 and above). You can try to apply it misting the leaves as well if you have foliar fertilizer. And for micro-nutrients, iron and (secondarily) molybdenum.
You can try that and check the new growth in two or three days. If that does not work, I'd re-pot it. And if that did not work either, as Cane said, we can open the official Mimosa Mystery thread.
Good luck,
Mandrake