How far does this power extend? Can they "cancel" light, or just create it? Can they create photons moving in a particular direction, or just omnidirectional bursts? How fine is their control? You're missing a lot of details right now, so I'm just going to throw out some ideas.
Humans, by themselves, are only sensitive to "visible" light (~400 nm to ~750 nm). If you want to be "detected" by a human without technological assistance, you're limited to this, and the recipient may have to be looking in your direction.
If you can create light far away from yourself, you can just create stuff right in front of the eyeballs of the person with whom you want to "talk". If you have enough fine control, you could "print" text, or even create images, practically right on their eyeballs. This would be relatively undetectable, or at least indecipherable, to anyone else nearby. If your control is really good, maybe you send a particular coded flash that's less subtle, after which the recipient closes their eyes and you create the image right on the inside of their eyelids (so it's much harder for anyone else to see even leaving aside being really tiny). Of course, if you can create light that's only going into the recipient's eyes, no one else is going to be able to see it or pick it up anyway, but "close your eyes so we can 'speak' privately" would make for good story-telling.
If you can't create light really far from yourself, you can create larger text nearby and hope your recipient is looking in the right direction. Range will be an issue unless you can focus the light into a laser-like beam, but if you can, you should be able to communicate with someone over a distance of a few km, at least. How far away from yourself you can create light will determine effective distance and/or whether you can create text/images or just pulses, since the angular resolution of human vision is limited. If you can also "cancel" light moving in the same direction, you can make this pretty visible under most conditions (but you'll still be affected by atmospheric scattering, so you need line-of-sight and smoke and/or fog are going to really impair this; rain, also, to a lesser extent). Otherwise it's mostly going to be useful at night.
Short version, though; if you have some decent spatial control, you can just make "text" (or images), and your recipient doesn't need to learn anything (assuming they're literate, anyway). If you're limited to a single point source, you can probably do better than fairly simple on/off systems, though you're still going to be limited by how finely your recipient can discern changes in intensity or frequency (and don't forget that a good chunk of people are color-blind, which is going to limit what you can do with frequency modulation). This is going to be a lot like learning a language, however, for both parties. OTOH, this may be common enough in your world that most people learn "light-speak" in childhood.
Now, if your recipient has a device with a radio, you can certainly send something like Morse code over other wavelengths. If you have really, really good control, maybe you can send at a much higher rate of speed, but being able to, say, spoof a WiFi connection is pretty unlikely. Even something like "proper audio" seems unlikely, unless you drag in the excuse that it's "instinctive", somehow.