Help with white growth on substrate

[Normalnot1]

My newish 5 gallon planted tank with a mixture of black sand and some kind of planted tank substrate that I can't recall the name of is growing a crazy white web of some slimy stuff! (The substrate bag is somewhere in my supplies room that has also become a storage room for my pregnant daughter's stuff)
The fish all seem fine, guppies, even some new fry!
Here are some photos. Can anyone help?


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[Normalnot1]

In reading another post, I'm fairly certain it is Eco-complete substrate. Thanks in advance for the help.
Oh! I should mention it seems to grow when the lights are off, and go away soon after the lights are turned on. I have LED's, but it's a low-light sit.


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[Barbz99]

It must be some sort of algae... I would just siphon it out of the tank and see if the growth slows down at al