All tho Limnophila is often categorized as an easy plant, but still needs a fair amount of light to grow healthy and dense.. And the turning point of just enough and just not enough is rather vague. As the whole concept of the lights intensity above an aqaurium is and it is difficult to express in term or number wat is enough and whats not.
For this you need experience, in knowing plants, how they grow and how they react to certain light and other invironmental conditions. This experience needs a lot of time, many plants and or a few tanks to learn.
It could be the start of algae, but still the trigger of it starting to grow there is in the plants health. And if a stemplant doesn't get enough light in it's deeper parts than the leaves down there will becomme unhealthy. Lose strenght and start to die off, melt and get susceptible to algae growth. And seeing how the plant in the picture is stretching out (long internodes/ long and leggy growth) it could be it doesn't get enough light. If it has enough light but grows leggy and developes algae it could be low co2 and to much nitrogene.
So stil from a picture alone, it is hard to say..Just guessing so far.. I guess probably not enough light for this type of plant..
(I wasn't even able to grow an already submersed form of Limnophila in my low tech tank, not even leggy, it just slowly died on me from day one, hadn't enough light for it)