The symptoms that you describe (no growth, light green leaves) actually sound more like a Nitrogen deficiency rather than excess. More information would be helpful.
A few questions for you:
Can you take a photo of your basil and post it?
What medium/potting mix did you use in the container?
Did it have fertilizer already added to the mix?
Is your "seaweed fertilizer" pure seaweed or does it contain other ingredients? Also, does it have the N-P-K ratio listed on it?
Pure seaweed contains only a small amount of nitrogen. It contains a bit more potassium, though. Excess potassium can block nitrogen absorption. Nitrogen is essential for plant growth & chlorophyll production and plants are unable to directly utilize atmospheric nitrogen.