this is a contamination, most probably from fungus.. discard the stock and filter all the media specially FBS. when you passage the cell line, make sure you used low RPM and wash with 1X PBS 2-3 times then replate it.

Thank you so much Sudipta. discard the stock(stock of what). I think repeated freeze thaw of FBS may also cause clumps formation.

Sumit Upadhyay The white substances appearing in your ARPE-19 cell cultures could be due to several factors, such as cell debris, protein or reagent precipitation, fungal or bacterial contamination, or issues with the culture medium. To address this, examine the substances under a microscope, check the quality of your culture medium and reagents, ensure that culture conditions are optimal, and consider subculturing the cells into fresh media. If the problem persists or is unclear, consulting with a colleague or cell culture expert may provide further guidance.