Does the black foam in terrariums/paludariums go moldy?

I'm planning on making a terrarium/paludarium and see a lot of others doing something similar use this black foam on the bottom, often acting as a filter for the pump for a water feature.

But I'm worried it would get moldy and need to be replaced which seems difficult under a terrarium!

My alternative thought is to use a hard water tight barrier (with expanding foam and sealant along the edges) so that the terrarium section can just use it's own normal terrarium drainage layer without worrying about water logging.

But with this I'm worried that the expanding foam won't be water tight enough, any advice with that is greatly appreciated too!