I recently heard what it is in one of Dire's episodes, it's a message from Mystcraft and it's from something that looks for stuff that isn't going to unload or something (it is related with the fact Mystcraft worlds aren't really permanent, so poorly coded mods can leave stuff behind in worlds that no longer exists or is abandoned, which can cause performance troubles/memory issues/etc.). The message is just saying that it detects stuff like that and on the worlds you actively use it shouldn't matter, the main problem is with worlds you don't visit or use anymore (ie, doing Dense Ore runs and then abandoning a world forever is what can cause trouble).
I think I also heard that REI's minimap is a culprit, but there are probably others as well.
On the topic of YouTube subscriptions, I personally wouldn't unscribe from someone who likes something I don't. You will never find someone who likes 100% the same things as you do, even with the people you know and like best you will have things you won't agree on. On the other hand, if someone want to unscribe it's their choice, regardless of the reason.