That's right - they do talk, or at least try to and it's your job as a the owner and keeper of your beautiful plants to understand the 'conversation'. Plants constantly communicate with their owners and relay messages according to their health. Sometimes you see this as algae, other times as poor plant growth etc. But occasionally they do something different and all of a sudden you see a nasty milky scum/film on the surface of your tank. Your natural reaction is try try and remove it right?
Sadly many see scum or surface film as a pest (which it is) and simply try and remove it with paper towers or water changes etc Whilst this might get rid of it on a short term basis (12 hours) it will come back for sure. The question is WHY and HOW do you stop it from returning? First you need to understand why it's happening. But lets see what it looks like first so we're singing from the same hymn sheet.
Looks pretty oily heh? Sometimes it's green too. Most people will have seen surface scum in their tank before - I have and I suspect you have too.
Cause - plant stress, too much light versus CO2 levels and nutrients. Mainly found in CO2 infused tanks but sometimes in non CO2 tanks with high lighting.
Remedy - reduce your lighting intensity and increase fertilisers and improve water movement. There’s been a lot of confusion over surface scum for quite some time, but it’s a fairly simple process over complicated on forums etc with hobbyists self diagnosing. The scum you see is fats and oils that are ejected from the plant whilst it photosynthesises and grows. The more CO2 pumped into the tank, the quicker plants grow and eject more waste. As you feed your plants more CO2, they need more nutrients, and therefore the balance between CO2 and nutrients must be on par with each other. If the lighting level is within the tolerance level for your plants, in some cases the plants adapt and no surface scum is seen. But if your CO2 levels are too low for the amount of lighting that you are pumping in, the plants will leach oils and fats into the water. Oil and water do not mix (we’ve all seen oil spills in the sea) and as it’s lighter than water it floats and this is the ‘scum’ you see. So in essence, your plants are giving you a very clear message as to what the scum is - you're stressing them out so you need to either reduce the lighting so it’s more on par with your CO2 output or increase your fertilisers.