Explanation:

Selenium (Se) is a necessary micronutrient. It’s involved in the amino acid selenocysteine (Sec), which is like cysteine (Cys) but instead of sulfur, it’s selenium. Selenocysteine is involved in selenoenzymes, which are important antioxidants. Oxidants are highly reactive oxygen radicals that are sometimes accidentally made by the mitochondria. They really like to oxidize and destroy things like proteins, fats, and DNA. As it turns out, we need those things to not be destroyed. Antioxidants are important because they’re useless and don’t do anything important other than be tempting targets for oxidants. They take the hit so you don’t have to. (though knowing biochemistry I’m sure that most actually do everything, because it would just make too much sense otherwise.)
Organic mercury poisoning is bad because it permanently inhibits your selenoenzymes. This leaves the cell vulnerable to oxidation which is particularly bad for your brain cells.
In this case, methyl mercury (Hg-CH3) has previously bonded to the sulfur in Cysteineball and has now come near the selenocysteine ball. Fun fact: the covalent binding affinities between mercury and selenium are about a million times greater than with sulfur. I’m not exaggerating, it’s literally cited as a million times greater. So yeah, Mercuryblob immediately jumps to seleniumball and basically never lets go.

Sources:

Ralston, N. & Azenkeng, Alexander & Raymond, Laura. (2012). Mercury-Dependent Inhibition of Selenoenzymes and Mercury Toxicity. 10.1007/978-1-4614-2383-6_5. https://www.researchgate.net/publication/278702735_Mercury-Dependent_Inhibition_of_Selenoenzymes_and_Mercury_Toxicity

Ralston NVC, Raymond LJ. Mercury’s neurotoxicity is characterized by its disruption of selenium biochemistry. Biochim Biophys Acta Gen Subj. 2018 Nov;1862(11):2405-2416. doi: 10.1016/j.bbagen.2018.05.009. Epub 2018 May 9. PMID: 29753115.