Some people really swear by chlorella, but it has caused major problems for others. Cutler doesn't recommend it as it's sulphur food which can cause mercury to move about a lot, ie in and out of the cells and drop the mercury without actually excreting it. It's not a true chelator as it contains just one thiol (a molecule that binds to mercury). A true chelator should contain two thiols (so bind to mercury much more tightly and escort it out of the body) so alpha lipoic acid, dmsa and dmps (all contain two) are the only true and thus safe chelators to use according to Cutler.
In the chelation groups on yahoo there are some people who have suffered very bad effects from taking chlorella. It didn't agree with me, made me feel very wired on very low doses.
Another reason is that some brands of chlorella are supposed to be contaminated with mercury as no doubt if it does absorb/bind to mercury to some degree it will have done this already from within its own environment.
Use this link for more info...