I'm not really sure WHERE this should go, so I'm putting it here. Photons don't count as particles, but they also don't count as walls, as such, if you accidently add photons, it's impossible to remove them.
...It's been that way for a while. Cracker seems to have fixed it somehow, but it was a glitch that came about from the original Powder Toy designer's additions to Photon behavior that he didn't seem to think would be simple to fix.
It's been like that for a while. Nothing to worry about.