One of the prime active ingredients is glutaraldehyde which is highly toxic. I believe hospitals use it to steralize operating instruments. Another ingredient is something called polycycloglutaracetal which I have no idea what this does. Unlike simple generic dry powder nutrient chemicals this stuff is like playing with nitroglycerine so it's not worth using the alternative. It's toxicity is one reason Seachem warns not to overdose (even though we do). It's another reason that it kills some plants as well as algae.
Wow it is some nasty s**t http://en.wikipedia.org/wiki/Glutaraldehyde
"A polymerized isomer of glutaraldehyde trademarked as polycycloglutaracetal by Seachem Laboratories, Inc. is the active ingredient in a product called Flourish Excel, a fertilizer for aquatic plants. It is claimed that it provides a bioavailable source of carbon for higher plants that is not available to algae. Though not marketed as such due to federal regulations, the biocidal effect of glutaraldehyde kills most algae at concentrations of 0.5 - 5.0 ppm. These levels are not harmful to most aquatic fauna and flora. Adverse reactions have been observed by some aquarists at these concentrations in some aquatic mosses, liverworts, and vascular plants"
So its not actually gluteraldehyde that excel has in it.