WS2812B LED, wrong baud rate?

I'm a little confused about how you are hooking it up.

So the WS2812B strip has... 4 wires coming out of the input end of it? Red/Black/Green/White?

As I read your post, you have it connected like:
Black - GND of PSU
Red - +5 of PSU
Green - B15/A7/B5 of Espruino (depending on which SPI you're using)
White - GND of Espruino

And Red and Black are +5v and GND (I would agree with this assumption) - and the others would thus be ground and data. Barring documentation to the contrary, I would assume that the white wire would be data, and green ground - it sounds like you're doing it the other way around.

You can, of course, check with a multimeter - either the white or the green should be connected to the black wire if it's wired the way we think it is.

(it sounds from your description like there's no chance there could be a fourth wire in the strip. If there was, it's conceivable that, particularly in a waterproof strand, they might run Dout from the end of the strip back to the start of it, to aid in chaining them. In this case, you would need to connect GND on espruino to GND of the power supply for the LEDs, and whichever wire was Din to the espruino)

Your issue is unrelated to the caps. The caps need to be next to every individual led (per datasheet), so the led strips and arrays already have the caps in them. You just need to think about the cap if you're working with naked 2812s.