What is this on my cucumber seedlings?

[Seanc]

Currently very low humidity <50%, little circulation as the grow room is in the basement. Here is a couple more shots.

[Seanc]

Here is another of a cucumber seedling.

[Seanc]

And another pic.

[GpsFrontier]

It's hard to tell from the pictures, can you wipe it off with your fingers, or is the discoloration inside the leaves.