This tshuva has a clear relationship to calling police for a sexual predator
Teshuvos v'Hanhagos (2:727): Question: A Jew is running a house of prostitution in the community, is it permitted to report him to the police?
Answer: It seems obvious that that if the reporting to the police closes the operation, then not only is it permitted to report it but it is a mitzva to do so for two reasons. 1) He is considered a rodef (pursuer) after illegal sexual activity (ervah). Just as with someone who is trying to kill another or even one who only causes another to die - he has the status of rodef. Here also he is considered a rodef after prohibited sexual activity (giloi arayos). Furthermore since many transgress prohibited activities because of him it is also beneficial to the community (migder milsa) to inform on him so that the receive a severe punishment. Thus it is permitted to inform on him ןif he is causing adultery and he will be imprisoned so he will no longer be able to do evil anymore.
However if reporting him does not stop his activities then it requires further analysis to determine if it is permitted to permit the very serious prohibition of mesira (informing) - even against such a evil person.
Nevertheless if at the time he is in jail the house of prostitution is closed down or its activities stopped or even if they are merely reduced - it would appear there is no problem at all to report him. This is true even if reporting him causes him to receive a long prison term - since today imprisonment is not considered life threatening. Therefore it is obligated to call the police.
Look at the Taz (Y.D. 157:8) who says a counterfeiter has the status of rodef because he is endangering the community and this applies to any other bad activities which are done on purpose. So surely in our case where he is endangering people's spiritual welfare. However it appears that even the psak of the Taz only applies if the reporting actually brings about an improvement. However if someone else will replace him and thus the activity is not stopped - he can not be reported for nothing as is explained in Gittin (7a) that even bad people are not to be reported to the government if it doesn't improve the situation.
This that you claim that only the house of prostitution is only for goyim. First of all it is also prohibited for us to cause goyim to commit adultery and to assist them to sin in this manner. He is also endangering the Jewish people because this is against the secular law. Furthermore it is a incredibly bad chillul haShem. In addition who is determining and guarantying that only goyim enter? Therefore it is obvious that this claim is not valid.
If it is possible to warn him in a manner which doesn't endanger the person issuing the warning then it is definitely obligated to warn him first. However if it is impossible because of the possibility of being harmed - then he is to be reported to the police without warning in order to stop the immorality and the chilul hashem according to the ruling of the Taz cited above.