It would become a meal...I've read that you can get away with some clown species, but that all depends on each mantis and it's personality!
You could always just get a very large clean up crew, I cant remmember the exact species of snail that targets hair algae but I know there is a few, and throw some medium sized hermits in there. They will control some of your algae problem and also add a mixture of food choices for your mantis!
If your tank is fairly new, less than a year, you can have hair algae problems, but just keep up a constant 10-15% water change schedule and you can control your algae growth a little better. Another great thing for controling algae would be to add a refugium to the tank and cut down the time your lights run (say 7-8 hrs instead of 10-12)
Good Luck