Day 2 of culture: Today, I observed black dots floating in the plate; none of them were attached to the Geltrex membrane, so they appear to be cell debris. I plated the cells with an increased density per well, which helped reduce the number of black dots. I know this because I plated a second pair of wells at low density and observed an increased number of black dots (floating). What did I do? I washed the wells with the media and replaced it without RI after 24 hours. Upon observation, there are now fewer black dots in the culture (in both high- and low-confluence wells). I will wait to observe the results after another 24 hours.

But right now, I know that the black dots are indeed debris, and at low density, the cells tend to release more of them.

Update: that is for sure: the black dots are indeed debris, and at low density, the cells tend to release more of them.

Be aware for cell density to start the differentiation, iPS to Cardiomyocytes generally need a confluence of 80-90% to start the differentiation process. If you have lots of blackdots in the well plate, pass the iPS to a new well plate (with lower dilution 1:6), to achieve the confluence in 3 days, then start. Avoid the blackdots (debris accumulation).