Flickering LED pool lights can be caused by the fixture, power supply, controller, connection or compatibility issue. Because pool lighting combines electricity and water, troubleshooting should stop well before a homeowner begins opening electrical components.
Observe the pattern
Does one fixture flicker or every light? Does it occur in one color mode, after the lights have been on for a period of time, or only when another piece of equipment operates? Those observations can help a qualified technician narrow the problem.
Compatibility matters
Modern RGB and RGB+W systems can depend on compatible transformers, controllers and switching sequences. Mixing components without confirming manufacturer compatibility can create unexpected behavior.
Do not treat underwater electrical work as DIY troubleshooting
Pool and spa lighting must be installed and serviced using the manufacturer's instructions and applicable electrical requirements. A licensed or otherwise properly qualified pool electrician should diagnose wiring, transformer and bonding-related issues.
For lighting selection, read How to Choose LED Pool Lighting for a Fiberglass Pool and browse Lighting & Electrical.