Odd 'web' over Sweetcorn seedlings

[teddybay-o]

All four of my sweetcorn seedlings have just appeared in my propagator. However two of the cells seem to have an odd web-like thing on them, spread over the majority of the cell. I'm not sure if this is a fungus, or something else, as my seeds were treated with Thiram to fight off fungus.

Has anyone else experienced this before?

The seedlings themselves look healthy just now but I'm concerned this will spread to other cells, and not just the sweetcorn.