Here's how to pronounce it:
Pla-seed
* Pla: The "a" sounds like the "a" in "father"
* -cide: The "ci" sounds like the "si" in "vision"
So, the "c" before the "i" is pronounced with a "s" sound, similar to the English word "decide".
Important note: "Placide" is a French word, so it's more common to hear it spoken in a French accent. The "e" at the end of the word is often silent or pronounced with a very slight "uh" sound.