I've never used Selcon and my peacock seems healthy. She's molted every 3-4 months, eats consistently, etc. As with human nutrition, a proper and varied diet for the most part should obviate the need for supplementation.
That being said, as long as you were reasonably careful (i.e. don't dump the bottle of Selcon into the aquarium), I would think it would be hard to harm the mantis with this stuff. It's pretty safe. It has fish oil (there's a joke in there somewhere), "marine lipids" (likewise), and vitamins C and B12, according to Marine Depot.
Dan