Not sure about plecos but snails tend not to eat live plants, so while they were munching on the plant, its more likely the leaves were dying already, and the snails can then "suck it up".
It would be useful to know whether the ghostly leaves in the pictures represent what remains of the original leaves, or whether they were leaves grown since it was installed.
Also, have you tried uprooting one of the plants to check its roots?
From the pictures, the plants look light starved, since it seems to have the materials to put out leaves, but not enough energy to generate chlorophyll. Lack of red is also a symptom of a lack of light. I'm not familiar with 3W LEDs. Does that even put out 120 mmol/m2/sec? If its far above the tank and the spot is "spread" too much, light intensity drops off significantly. Your pictures look really dark, but hard to tell given light sources and camera equip.
I grow a small rosette sword (E Parviflorus) in my 10g nano, injected CO2, under 48w CFL, 2 dKH, 6.8ph, 1 ml of Seachem Flourish /wk, and it looks to put out a new leaf every couple days. New growth is a little transluscent but not sure if that is just a growth habit or a deficiency. It sort of fills in with the green in subsequent days.