The scenario you describe involves a researcher observing living neural tissue under a powerful microscope and noting the connections of some neural cells to capillaries and their role in nourishing and supporting other neural cells. This would be considered a naturalistic observation because the researcher is observing the behavior and characteristics of cells in their natural context without manipulating any variables or conditions.
A case study, on the other hand, typically involves an in-depth investigation of a single subject or a small group of subjects, often including extensive data collection through interviews, observations, or other qualitative methods. In your example, the focus is on observation rather than in-depth analysis of a single case.