@MTUFC
Could be a lot of things, but maybe a modified 'fog on the streets':

Fog on the Streets:

Reduce the '<Fog value=' to zero for each weather in 'weather.xml' to test that:

'weather.xml' File Location:

...\mods\update\update.rpf\common\data\levels\gta5\weather.xml

Example:

<WeatherTypes> <Item> <Name>EXTRASUNNY</Name> <Sun value="1.0"/> <Cloud value="0.150000"/> <WindMin value="0.0"/> <WindMax value="0.0"/> <Rain value="0.000000"/> <Wetness value="0.000000"/> <Snow value="0.000000"/> <SnowMist value="0.000000"/> <Fog value="0.000000"/> <!-- Set this to "0.000000" for each/every weather in 'weather.xml' --> <RippleBumpiness value="0.420000" /> <RippleMinBumpiness value="0.250000" /> Thoughts/Diagnostic Approach:

I don't know Redux, but if it supplies any scripts with it, try test removing them by removing them from or renaming the 'scripts' folder in your GTA5 root folder (if you have a 'scripts' folder there, not always the case) & loading the game to see if the effect is removed etc.

If it's neither of them, you're probably looking at editing 'core.ypt' to remove the ptfx fog from that effect. Depending on how it has been done, simply making the texture transparent might also remove fog/smoke from other parts of the game (lots of different effects usually use the same ptfx textures etc).
If it is a 'core.ypt' ptfx, ideally, you'll want to export 'core.ypt' to 'core.ypt.xml' using Codewalker's right-click 'Export XML...' functionality & edit the file to only remove that effect while leaving all the others that use that same texture untouched.

'core.ypt' Edit Test:

Use this edited 'core.ypt' to ascertain if it is getting it's texture from 'core.ypt'. I basically made all the fog/smoke textures transparent. If Redux has added new additional texture/s to it's 'core.ypt' for that effect, it/they will be missing from this one. If that's the case, it'll probably look weird in-game, but could be invisible also, or even cause a crash (I'm really not sure, hence, make sure you create a full 'update.rpf' backup first before doing anything ).

Installing 'core.ypt':
After making a backup of your modded 'update.rpf' install that 'core.ypt' here:

...\mods\update\update.rpf\x64\patch\data\effects\ptfx_lo.rpf\core.ypt

here:

..\mods\update\update.rpf\x64\patch\data\effects\ptfx.rpf\core.ypt

& here:

...\mods\update\update.rpf\x64\patch\data\effects\ptfx_hi.rpf\core.ypt

then load up the game & see if the fog has been removed or the way it looks changes in any noticable way.

See how you get on with that anyway, hopefully it's just 'fog on the streets'.
I can help with 'core.ypt.xml' editing if it comes to it, but we'll need to figure out exactly what texture it is using first, let me know how you get on & what you discover & we'll take it from there