Yes, if a toilet overflows and water seeps through the baseboard, it can potentially cause water damage on the ceiling of the floor below. If the water remains stagnant or is not promptly cleaned up, it can lead to the growth of mold or mildew, which can further damage the affected area and pose health risks.
Opening up the ceiling to check for mold would be a reasonable course of action if you suspect water damage or mold growth. By inspecting the ceiling, you can assess the extent of the damage and determine if mold is present. If you discover mold, it's essential to address it promptly to prevent further damage and potential health issues.
If you're uncertain about how to handle the situation, it's advisable to consult a professional, such as a plumber or a mold remediation specialist, who can assess the situation and provide guidance on the necessary steps to take. They will be able to evaluate the extent of the damage and recommend appropriate actions to repair any water damage and address any mold issues.