oh my god!!!youv gotta be kidding.thats full blown plant aids bro.your plants been havin it off behind your back without protection.sorry dude,i really feel for ya.
seriously though what madbud said is a good idea.remove the leaf and put it in a zip lock bag.just curious what does the underside look like?looks kinda like a bugs made a home under the leaf and I'm just wondering if underneath the bugs encased in silk.
Looks like some sort of cocoon or egg sack... The others are right Quarantine that thing until you can ID it, just in case it decides to hatch & invade... Plant aids ... too funny...