It's often bacteria but without seeing a photo I can't say for sure. It can be the results of high nutrients and sometimes shows up in tanks with low flow. A little isn't a worry but in large quantities it can be a real problem.
The only slime I see is on the top of the heater, The filters flow is quite powerful that I have to keep it down. I only have a 54L tank. The only fish I have in it at the moment is a bristlenose pleco.
I'm going to guess bacteria. It's possible that it is warmer and more still by the heater (the heater itself can act as a break for flow). I do see what wrighty was getting at. It could be eggs of some sort too if there's anything like that in the tank. Can you post a photo maybe?
I can't see it but just based on location and what I do see I'm sticking with my bacteria. It's all over your tank but it's just found a place to flocculate.