I think I have the answer. I stumbled across your post and this board while trying to find out what the critters on MY glass were.
I have a 130 l freshwater tank that was recently decimated by some apparently healthy quarenteened but ich infested fish I bought along with a java fern.
I've continued the ich treatment for weeks because I thought the things on the glass were ich trophonts but having borrorwed an eyeloop and and a digital microscope camera...I can show you the ugly brutes.
I think they are freshwater limpets - acroloxus lacustris which must have hitched a ride on the java fern and multiplied in number as my fish died of from the ich. I curse the fish shop to hell. I washed the fern what's more. I'll have to go back to soaking new plants in anti snail meds.
Anyway now I have to find out how to get rid of them without torturing my remaing cory and ottos any more (they haven't enjoyed the ich meds, higher temp, salt and so on).
HTH
Scrumpy
PS Pic from the web first....having trouble uploading mine.