Good point.
Not that I'm a medical doctor or anything, but taking normal level of supplemental iron should not cause most people any problems. Though I doubt it is needed or very helpful, especially in regards to most males. But if I know anything about Bowser, it's a good guess that he has tendency to over do things. Taking too much of any supplemental is probably a bad idea.

And as far as putting tens of tonnes of iron in a ocean, this is example of not very much. If it was instead thousands of tonnes that might possibly be a different matter- though probably even in these higher quantities there is little risk of any significant consequence- a volcanic eruption could and has in the past easily put 10 or 100 or million times this amount iron, not mention some other rather exotic stuff, into the ocean.