I agree with Chickenheadknob.
Look at the light-blue cap on the 5th photo. It has a curtain of this coating hanging between its legs.
Xenon has a point about the "curtain." You might want to probe that with a toothpick. Maybe record a video of it, while you poke at it (gently,) because you are likely only going to get one chance at it. Meaning, if it all crumbles off at once, and you are not sure what you saw, you could review the video. If it is a curtain, then I vote for it being a coating.
You could also test the substance with plain water on a Q-tip. If it is leakage from an e-cap, plain water should get it to at least move/smear. If it is a coating, it should not react at all.
You often hear about e-caps leaking. But, I have seen them out-gas, too. This is a little different than them spewing their liquid contents, due to the build up of internal pressure. Out-gassing is more like a smoke, or fog. It can coat components on both the top and bottom of the PCB. It can also have a limited pattern, caused by airflow in a particular direction. Out-gassing + humidity + heat = bubbling = Maybe?
If it is out-gassing and you don't find a bad e-cap, it is possible someone changed the cap, but didn't clean the board.
If it is sticky, might it be a spilled Sprite/7-Up?
I hope that gives you some ideas to think on. Please do post your final determination of what it is.