I have 1 piece of LR that was only about 90% cured and it is covered in it. It's not really foamy, its more slimey and soft. I posted as to what it was and got same answer as you did.
Without a pic and from your description, I would say sponges also. They like the dark or shaded areas of your LR, so that is a perfect spot to find them. They are harmless filter feeders as said above. If you can get a pic that would help. HTH