Symbols are shorthand for messages, and can mean different things to different readers. For example, an upside down cross is either a message of anti Christianity, as inverting a symbol implies distress or disrespect; or it is a symbol of reverence. St Peter, according to legend, asked that if he be crucified that it be upside down, as he was unworthy to die like Christ.

I bring this up because "It was written by migrants as a threat" is definitely possible, but does not seem plausible with the information I have at hand.

A Nazi sprays a swastika on a synagogue to incite fear. Every Jew knows that symbol. "18" is sometimes used as a subtle dog whistle between Nazis. Spraying that on the side of a synagogue would be pointless. Nazis know it's a synagogue, the Jews know it's vandalism, but the 18 doesnt have the threat associated as inherently.

If this is a threat, it's a silly one without context. This seems more like a symbol of rememberance. It means something to someone. Whether it is "Remember our brother who died" or "Avenge what happened here" or "We did the thing here". The latter seems unlikely if it is being sprayed well after the crime.

Per the symbol itself, Byzantine crosses often have the angles foot stool at the bottom. The Cross having some Christian connotations seems likely, it being such a common symbol.

The circle at the top immediately invokes thoughts of a crown of thrones to me, but I can't say for sure, again symbols have meaning in context.

It's interesting that the dot is inconsistent. I don't think it being there or missing has extra meaning. That seems like a too subtle a detail to be part of the message. Which makes me think that it is a mistranslation/ miscopying. Either the original had a dot and the copier didn't see it, or thought it wasn't intentional, or the opposite. Maybe the original had an unintentional dot, but the copier copied it verbatim.

Which would make me think the dot, while not necessary for the message, can be interpreted as part of it, even if it wasn't the original intent.

It can also lead to the idea that the person's doing this aren't in constant communication, otherwise someone would say "Why did you add the dot? Don't do that".

It may be a message of hate and warning, but I'd be hesitant to spread that fear mongering without more evidence. Both for maintaining peace and to keep it from being adopted as a message of hate if that is not its original intent.