It is certainly true that ICP does not distinguish chemical form and, depending on how it is performed (prefiltration, centrifuging, etc.), may not distinguish very fine particulates or colloids from individual ions. Seachem made a big deal out of this as well. In that case, I filtered samples to remove particulates above 0.2 microns or so, but particulates below that size may still exist.
I do not recall ever claiming that Marinepure was toxic or negatively impacted organisms, but the characterization of particulates as "inert" implies a level of understanding that is not there. Many "inert" particulates that do not dissolve can cause serious issues to people. Asbestos, silica dust, etc. There's no a priori reason to assume particulates could not possibly impact organisms such as filter feeders, whether they might clog up the mechanisms (such as sponges).
Microplastics in the ocean and its ability to cause problems for organisms is an active area of research in many labs. Thus, an "inert" particulate is not a get out of jail free card.