Aquariums: What’s this white “fuzzy” stuff on the roots of my water lettuce?

Got some new water lettuce from the local shop (very reputable) about 10 days ago. split it between two tanks. in the larger, 20 gal, tank, I just noticed some white stuff growing/gathering on the roots of the water lettuce. The water lettuce in the other (6 gal) tank is fine.

Water levels are normal, have some fish, snails, and shrimp in it.