I think it is red planara (I think that is how it is spelled lol) flatworm. This is why we dip our corals and quarantine them. I am not sure if there is a better option today but in the past we would use flatworm exit to get rid of them. You want to do this asap, because once they multiply you have to take precautions in getting rid of them otherwise you could kill your corals. If there are not to many of them then I would just vacuum them out. There could be better solutions in 2022, so wait to see if anyone else chimes in.. Good luck
No corals yet. I started the tank in February. Lost the two damsels in there due to ich. So I spent time building up clean up crew and fighting off the dinos in there. I have one more round of cuc to add today then I'll start the fallow clock and quarantine the fish this time. Add corals after that, but plan to quarantine them too.
Most likely a Convolutriloba sp. of flatworm. Generally harmless, but can become a nuisance if they become abundant. I've always had a handful in my systems and they've always been fine. If you start seeing the red ones, they're more likely to be problematic, but the clear ones are generally benign.