This sort of thing should be a cakewalk that I've done plenty of times...or so I thought.
So I do exactly this:
Create a new map.
Create a new Effect (of type Damage) with an area, and with a search filter that excludes Self, Player, Ally.
Create a new Behavior (of type Buff) with that Effect as a periodic Effect every second.
Give the marine Unit that Behavior.
Run the map.
Rip in peace marine.
Why are search filters not working in this case? If this is working as intended, what am I missing? It seems like the Effect is totally "neutral" with no player affiliation. I tried fiddling with the Behavior's Player field. I tried using a Validator in the area search (which is just another filter). I tried using Search Area + Damage Effects instead. I tried a few other obscure things.
Yes! I must have overlooked the Effect's Target: Impact Location field for some reason. It works when I change it from the default Target Unit/Point to Source Point (oddly, without any search filter). Thank you!