^^It's a mutant branding from traveling to a possible future,, the same way Bishop got his mark, although Bishop didn't time travel there. I'm pretty sure that's also when Jamie Madrox got his mark.
The mark bishop has is just a mutant brand that was given in mutant concentration camps in the future. Madrox and layla went to the future together to try to find out who was the betrayer of mutant kind in messiah complex. In the process they end up stuck in this concentration camp where their branded. Thats where they learnt that bishop was the traitor. Madrox ends up back in the present but layla is stuck there from 15 years.