Help! What's This?

I don't know whether this is pest or disease. But I found it under a geranium leaf yesterday.

The plant has recently been over-watered, so it could be the disorder oedema (corky scab).

However, it seems to be mostly on the veins, which would tend to indicate pest trouble. Or perhaps it is scar material from a pest attack (though haven't seen any pests)? Can anyone help?