Those are
Ostreopsis spp. dinoflagellates, so the behavior you describe matches the literature.
I always recommend a collection of approaches because the feedback on how people successfully dealt with them is very mixed.
First I would try the very opposite of what most people recommend, but it's still working for some: Cleaning up the tank. Water changes, cleaning the substrate, making sure there are no dead spots without flow, etc. Many people associate low nutrient levels with dinoflagellates, but whether they are a symptom or a cause is not fully decided, and I personally think both might be possible.
If that doesn't seem to improve the situation after a few weeks, and especially if you had low or undetectable nutrient levels
before the dinos appeared, I can recommend ammonia and phosphate dosing depending on what your tank may need. I can't recommend overfeeding as this introduces unwanted organics, so I would follow the instructions below instead:
https://www.reef2reef.com/threads/diy-ammonia-dosing-for-low-nitrate-systems.987087/
https://www.reef2reef.com/threads/diy-phosphate-dosing-recipe.1109919/
For
Ostreopsis, a UV sterilizer might be able to help, so try to find one that fits and make sure not to push more water through it each hour than recommended or it won't have the same effect. Here is an interesting thread on the issue:
https://www.reef2reef.com/posts/14480797/
Silicate dosing is another approach many people try, and it certainly won't hurt if the problem persists. I'm skeptical about bottled bacteria products, but as some people seem to have made progress this way, it might be worth a try. Just note that these products can be nutrient bombs and might feed the dinos.
Hydrogen peroxide dosing is quite experimental but there are some positive reports. I'm sure you will find appropriate instructions for reef-safe dosing somewhere. Silicate dosing is another commonly used approach to encourage diatom growth and have something compete with the dinoflagellates over light/space and nutrients.
If all this doesn't help you can try something like Fauna Marin DinoM (formerly DinoX) or other products, but I would only do so as a last resort. There also is the UV sweeper which helped some people but I would ask "Moe K" about that once you have exhausted all other options. They are very active here on the forum and may have additional ideas for the treatment.
