I'm in cedar park, let me know if you want a second eye to make sure, but that looks like exactly what I had which were dinos.
After re-reading your post.
I would suggest reducing feeding all around.
example, do the nori every other day opposite of feeding days.
scratch the pellets or flakes totally (lots of phosphates in dried foods)
and raising the ph.
Definitely worked for me.
ur on the right track otherwise.
took about a solid 2 months until it was 100% gone for me.
How are you siphoning it out? I used a tube with a sock tied to the other end, and started the siphon with a return line nozzle to create a strong siphon. they stuff is in the rocks, not just the outside. (when I wasn't replacing it with clean water)