The predator's eyesight is a continuity error in the first place. In the first movie it was like a very washed out thermal sight, in the second it was clearly thermal vision. There is a theory that the first pred was colour blind, but then technically he wouldn't have been able to make out what was going on with his vision modes even with his mask on.
Aliens can still be seen in thermal vision, just not very well, their bodies would show up as a negative heat source, meaning that if they were fighting in a hot environment, the predator could see the alien. At the beginning of AVP, in the extended version, when u see the last survivor of the whaling station being chased by the pred, & then an alien leaps out, u can actually see the alien fairly clearly in thermal vision just before he changes over to EM vision.
So even without their masks, predators can still detect an alien, they can still somewhat see them, & their other senses would be put to good use also, I'm sure. Although the mask is a definite advantage, I severely doubt the pred would need it to win a fight, especially an elder.