Steve,
I would think the iron would show up on the TDS meter....I know I went from 0 to over 20 while making only 20 gallons. The pump problem really knocked out the DI resin fast.
FWIW...Julian Sprung wrote this about Dictyota in "Algea, a problem solver guide."
"Not palatable to most herbivorous fish. Fast growing. This alga blooms in aquariums under good water quality conditions, and it seems to have a positive effect on water quality, though it may shade or smother corals."
control by herbivores and by hand. He says that the mithraculus crab (emerald green guy? perhaps?) and a Diadema Urchin will eat it.
My crabs never touched it, but then again, it was growing on the Seios.
Maybe something else has had some luck with other critters. Good luck.