Have I become a steemit Cooler?
I've focused my efforts on "curating" content recently, looking through introductions, articles, and art on the platform and up-voting the things that I find interesting. Unfortunately, it looks like my up-votes are actually costing people a few steem cents? Can this happen? I've noticed a time decay in the rewards - basically if you don't keep receiving more votes and traffic, the reward decreases VERY slowly. Is the decay large enough that in the time is takes me to read a post and click on the up-vote button, the decay is larger than the value of my up-vote?

its the price change on what steem is worth in the internal market. It fluctuates every min to hour.
Thanks for that, very happy to hear I'm not crashing the party!
Yikes! It's this in the documentation anywhere?
Fortunately it's not really an issue, have a look at the explanation from @karbonxx above (and give him an upvote for it!) I didn't realize that even though I was adding to the original poster's total reward in terms of steem power, the value declined somewhat overall due to changes in the internal market.
Keep up the good work!