I conducted monoclonal screening of the NCI-H460 cells, and planted 5,000 cells in a 12-well plate. After 24 hours, the cells adhered to the plate and were in good condition. However, when I observed the cells the next day, I found suspected pollutants on some cells, which adhered to the cells and appeared many small black spots around them. Has anyone else been in this situation? Is this cell contamination? What kind of pollution is it? Looking forward to your reply.