No, it isn't. It's obviously sand/dirt. I am sorry that I guessed at that. Experience has taught me on this forum that that issue will be algae 95 times out of 100 but I guessed nevertheless. I would not pursue "algae" as your problem.....it is sand.
So, if this sand suddenly appeared a weeks or so ago, can you recall any changes you made to the pool at that time? It looks too coarse to be blown in.
Curious.....why are you calling it "silt". Silt is powdery and close to mud........your pics look like clean washed sand.
No problem at all. I appreciate the opinions and it just helps me check boxes of what it is and isn't. I didn't go through with the SLAM so no harm done. And eliminating any concerns about pool chemistry allowed me to focus on how this material is getting into the pool at the same time every day in that unusual location. It didn't look sandy at all in the pool, only after I sampled it and probably washed most of the fines away. It is a very small granule sand left behind.
Anyway...with all the pool chemistry boxes checked I woke up this morning with a theory that seems now completely obvious (aren't they always?). It is an irrigation leak, somewhere under or near the far edge of the deck. I turned on the drips and sure enough after a few minutes out pours the dirty water between the deck and the tile (see pictures). The timer comes on just after the pool pump finishes it's cycle, which is why it wasn't getting cleaned out. In a strange coincidence that I didn't put together until this morning, the breaker to the irrigation timer must have tripped the same day I changed out the filter cartridges, which is why the dirt magically went away after the filter change and didn't reappear until I found the tripped breaker a few days later after the lawn started drying out.
So much relief knowing it isn't a pool issue. Not as much relief having another irrigation leak to take care of. I'll get the irrigation fixed and hope it didn't scour too much under the deck or near the pool edge.
Thanks to everyone who took time to respond and help me think through this mystery.