Q.Something ate all the leaves on my Cassia over the last 24 hours
I am attempting to grow a cassia from a cutting. It successfully has gone from a cutting to a small plant with a good root network, out growing a 2 gallon pot. I transferred it into a larger pot, where it continued to thrive. Tuesday I noticed a small something attached to the underside of one leaf (white, looks like bird poop) which I assumed to be spider eggs or other insect, which I left to monitor. Thursday I come home from work and All of the leaves from the 14″-16″ plant have been eaten. Any ideas of what gives?

The white blob on the underside of the leaf may have been an egg case from an insect pest, maybe a caterpillar; without a photo we can only guess. Spiders wouldn't defoliate the tree.
Next time you see a suspicious object like that, don't "leave it to monitor", wipe it off with a rag or cotton glove, before it can mature and do any damage.
It's possible that the white blob is not related to the pest that defoliated the tree.