Probably alge..Don't see that often with perlite and should not be a problem. usually caused from light and or, not having any air moving. I would get a fan going and you can put a layer of soil on top if it bothers you.... They look ok for clones and if theirs healthy new growth should be fine...
Have surface alge on the coco every grow in my room. I use Botanicaire Coco with perlite & water once a day under a 600 watt light. It has never been a problem. Cheers