Chlorella is not a true chelator. Chlorella is more like a chelating assistant. It helps by grabbing loose mercury out of your digestive system and carrying it out of your body. It doesn't work if the mercury is stuck in various locations in your body rather than floating freely.
A chelator is something which pulls heavy metals out of the places where it gets stuck. Chelators are things like DMSA, ALA, cilantro, DMPS and glutathione(your body's natural chelator). A good chelator will also pull mercury out of silver fillings and into your body. This can make a person very sick.
Using chlorella by itself is not a bad thing (it's got a lot of vitamins in it), but it's not the same as really chelating.
I suggest that if you are serious about chelating, you try the Cutler Protocol *after* you replace your fillings. Cilantro and chlorella is another method. I can tell you that the cilantro method made me feel better after 6 months of using it, but can have a lot of side effects. Cutler has worked too without as many side effects.