I built a slime farm before realizing that an underwater monument is in the hostile mob spawning range of the farm. The massive amount of guardians that spawn are affecting the efficiency of my farm. Is there any other way to stop them from spawning other than draining the monument? Does removing the entire monument stop them from spawning or do they spawn in whichever chunks the monument was in? If I could get any help on this that would be great, thanks.
Guardians spawn between Y=39 & Y=63 [or top of sea level] in the 58x58 footprint of the monument, and continue spawning in water (source or flowing) regardless of the presence of the monument strcture. [The spawning block and the block below must both be water (either type) and the block above must have a zero collision mask (eg. air, water, sign, open fence gate)]
OP does not mention version, but light level has no effect on guardian spawning (unless this got slipped into 1.14); as mentioned, skyview does, decreasing spawning (by a factor of 20).
Depending on the spatial relationship of the monument and slime farm, it should be possible to arrange the kill room/collection point for the slime farm to be on the 'other' side and quite low in the world, both of which should help in getting the guardian spawning volume outside the 128 radius.
Worst case would be to fill the nearest part of the monument (likely something you'll eventually wish to do anyway) as it sounds like the monument is fairly far from the farm… (You could also just accept a small loss of efficiency in the meanwhile…[Provided the monument is near the edge of the spawn range, the number of spawnable spaces and the time before natural despawning should be fairly low.])
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.