Is it possible to eat "self grown" phytoplankton by the pound like whales with no worry of iron and other minerals building up in the body and mind?
Is it impossible to overdose on natural organic minerals?
Your real question here seems to be if the human body can take in too many natural vitamins or minerals to be toxic. Remember that plankton are microscopic creatures and gathering together a pound of them would be an enormous amount relative to what a normal human could possibly eat. Whales are orders of magnitude larger than humans in mass, hence the reason that they are feeding essentially constantly.
It is possible to overdose and consume toxic amounts of anything, including water. However, there are mechanisms in place that make it difficult for a human to overdose on vitamins and minerals through the course or normal eating. In the case of iron, the human body only absorbs about 10% of the iron ingested, and once ingested it is used in things like hemoglobin to help transport oxygen through the bloodstream. A lack of iron in the blood is called anemia and an excess of iron accumulated in tissues is called haemochromatosis. It is usually only found in people that are genetically predisposed to accumulating too much iron in their tissues including liver, heart, and pancreas, thus damaging these organs.
The bottom line here is that a toxic overdose of vitamins or minerals is unlikely through the course of normal eating. Only when taking an excess of supplements can this start to become a problem.