/fill with blocks added by a mod
Solution 1:
Thank you for that comment, @Reivilox.
The mod id is 'colored_water', one of the blocks is named 'glowing_water_green', and you can /fill with this block using:
/fill x1 y1 z1 x2 y2 z2 colored_water:glowing_water_green
Solution 2:
Well I figured this out on my own so here's an example from the Chisel mod: chisel:basalt2 1 so basically you /fill cord cord cord and then the name of the mod with this. In my case it would look like chisel: then the name of the block and whatever variation of that block it is.