“Wicop” says that there should be no solder resist between the pads of the LED. They say that it is because flux residue can cause short circuit between the LED pads and the flux residue more readily settles on top of solder resist.
Most modern solder pastes leave almost most deposits behind. Pay a little more for one that is of high quality.
I think there is more to this than the manufacturer is letting on. I suspect the main reason the manufacturer does not want any solder mask between the pads is because it raises up the part on the board. If the solder paste is not thick enough it is possible to get open circuits (which is our experience), not short circuits.
Anyway, you can minimise the likelihood of open circuits by having a thicker solder paste stencil or minimise the likelihood of short circuits by having a thinner solder paste stencil. You can also adjust the openings in the stencil for the best reliability.
Give it a try. You have nothing to lose & everything to gain.