Did you use water retaining granules when you potted them up ? If so, this is probably some of that, it has swelled up, and that is what it looks like when in the soil.

cinderella's suggestion is the most likely one - otherwise it could just be algae or slime mould.

Yes, probably water-ret. gel. It stays small during summer (too dry I suppose) but as soon as the autumn rains come it swells up (each piece can be the size of a hazelnut) and slimes it's way up to the surface of the compost. I just brush it off!

I just push it back into the compost!