Here is an old-ish article that talks about similar white residue
https://www.edn.com/flux-residues-can-cause-corrosion-on-pcb-assemblies/I started having this problem some years ago with PCBs coming back from a new assembly house a client used. Apart from an aesthetic issue, the residue was apparently sufficiently conductive enough (or hygroscopic) to cause problems in some of our high impedance designs.
The white residue can't easily be removed by IPA, as it is the wrong solvent. Water washing may also not work by itself depending on the type/mix of fluxes used. I found IPA could work with extensive scrubbing, but that is hard if not impossible with lots of small passives and/or fine pitch devices. Something like Ultrasolve (Electrolube ULS) worked better than IPA, but was still not great and incompatible with some parts.
After another recommendation here back in 2015 I started using Safewash Super (Electrolube SWAS) as mentionend by IanJ. It may seem expensive compared to other cleaners, but I found it works really well for me and is well worth it.
I filled up an small ultrasonic cleaner with it (don't recall if I diluted it), and usually run it for 180-300 seconds depending on how many PCBs I load up. SWAS is a saponifier so the solution and PCBs will feel very soapy, and you need to rinse the PCBs well after cleaning. I initially use tap water and then rinse again in deionised water. I then dry the PCBs in a low temp oven. I don't have particularly large production runs, and generally only put my smaller PCBs through this cleaning process.
Interesting what IanJ mentioned... I have had the same batch of SWAS sitting in a dedicated ultrasonic cleaner for several years, and although it has turned a deep green colour it still appears to work fine. I should really start a new batch considering it comes in a 5L container and it has a limited (4 year?) shelf life. I avoid using heat with it as I recall the data sheet says it decomposes at lowish temperatures, I think 45C max is recommended. Also it is a skin irritant, so best to wear gloves. Read the TDS & MSDS!
Note that some components (particularly electromechanical) should not be immersed in liquid, or may be damaged by ultrasonic exposure. This may not be evident immediately, but lead to premature failure. Check the component manufacturer data sheets & published advice.