Hi Sam, this is a pic I took from the belly of my sulcata. Is this a concern I need to deal with? Or is this just from all the rain we have had recently? Thanks for your expert advice!
I believe you should treat that.
That is a Scud Fungus; it slowly grows on their shells eating away at them. Most times it is just defiguring, but sometimes it can become pathagenic.
Clean the shell with soap and water.
Scrub with Betadine and a soft wire brush. https://www.harborfreight.com/detail-brush-set-3-pc-69638.html
Spray with Blue wound spray.
Scrub, trying to work on the black crud, or at least until the dead black shell is removed.
You can go slow and do several treatments, you don't have to do it all in one day.
Dry and spray blue.
More information in my care sheet "Shell conditions": https://www.floridaiguana.com/caresheet-aldabras