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looks like Coraline algae starting to form , I've seen it in red , purple , orange , yellow and blue sometimes even in white , it is harmless nothing to worry about ,
Looks like sponge growth, see if you can just wipe it off. Won't hurt anything either way. New tanks with high phosphates will grow them more than others.
If it is hard it would be a type of coraline algae.
If it is soft it is probably the start of a sponge growing...though you don't typically see them growing like that right out in the bright lights but will find them in the shaded areas most of the time. There are exceptions to this rule, but those sponges are rare, but beautiful.