It could absolutely be brown algae. Unless you have silicates in your source water (like me) diatoms should disappear on their own. Be patient, the time with diatoms varies.
Although now that I look at the other pics, it does look like fish poop stuck on the leaves as well! How did it get way up there? Doesn't look like
nerite poop which is usually short little turds.
I didn't Google
Microbe-Lift, but it's most likely a bacterial supplement. I believe in them, I believe they help with diatoms and fish poop too.
Read this if you haven't already, I use SepticBac for $2
The Use of Probiotics in
Aquaculture
If that's poop on the leaves, why is it landing there? Flow? Don't worry about questions, that is how you learn, along with research, but you have to be careful which sources you get your info from, there's a lot of untrues out there! I would try to remove the poop daily by wiping the leaves lightly with a Kleenex. Mechanical damage will cause dark marks on the leaves. Then figure out why poop is accumulating there. I used to have my
pennywort leaves on the very surface, the top of the leaf was dry, so no diatoms. My pennywort had since melted away, so if it's growing, don't give up on it, it's such a pretty plant
Large Nerites might have trouble attaching and balancing on a pennywart leaf, they might be too heavy for them. TIP: Just fyi, maybe you know this, if a nerite falls upside down, it will die since it cannot right himself back up. So if you see your Nerites on their back, turn them over upright.