The culture is most likely contaminated with bacteria or fungi (yeast).

• Bacterial contamination typically appears as scattered small particles and aggregates.
• Fungal/yeast contamination usually shows up as more circular and compact colonies.
• Check for turbidity and odor in the culture medium.
• Perform Gram staining or subculture on an agar plate to identify the contaminant precisely.

May I know what kind of cell this is? Is it a primary cell obtained from tissue samples, because the background looks like it contains a bunch of other things other than the cells themselves? How was the colour and turbidity of the culture medium?

The cells in question are N27, an immortalized dopaminergic cell line. The medium appears to have a yellowish color.