I guess you need more of a flow/circulation, some kinda water movement. In general the boundary between plant leaves and water is static (i.e. no water movement). The frictional effects and boundary layer issues between the two mediums always end up like this. (Ever wondered why there is too much dust accumulated on the blades of a table/ceiling fan, even though the fan is rotating at Jet speeds??)
Since the nutrient and water exchange occurs only via this boundary, without any flow in the tank, a sloppy/slimy stuff starts appearing. Nutrients cant be exchanged, the pores get choked up, osmotic issues lead to wrong way water movement from the plant to the tank making plant de-hydrate, This also increases the bacterial/algal activity that thrieves on the nutrient buildup outside the leaf. Leaf gets less of O2 and also CO2... All this leading to what you have been seeing in your tank.
Simply clean up the mess and put a small water moving element. If the tanks a really tiny nano, a small DIY rubber stirrer strong enough to do this would do the trick (something like a mini hand-held USB fan type???).
cheers
niru