Various symbols appear throughout the Slender Man Mythos, serving many different purposes. The most well-known of the symbols is the Operator Symbol.
The Operator Symbol is a circle with an "X" drawn through it: Ⓧ. It has not been revealed what it represents or what its function is except that it is somehow connected to the Slender Man/The Operator. It was first depicted within the Marble Hornets series and has since appeared in many other Slender Man series as well, including TribeTwelve and DarkHarvest00.
Since it was used in Marble Hornets, its popularity has led it to be synonymous with Slender Man by the online community.
This symbol has been used as a logo by EverymanHYBRID since their early days as a fitness program. Unlike many others, however, it is not known to be connected to the Slender Man or any other supernatural entity. A box, left by HABIT has been found with this symbol on top of it, made of purple tape.
HABIT reveals that the symbol commonly associated with the Observer (and, ostensibly, the Collective) is actually a rune of great importance and power. It symbolizes "severance", and acts as a separator between someone's or something's and a specific object—placing it on an object severs ties between that object and its owner or controller. It is stated that, in many cases, the Observer used this symbol to hack into Noah's computer and his various online accounts, by 'separating' them from Noah's control. HABIT explicitly states that applying the symbol to a weapon would turn it into something that would harm the Administrator, or at the least free Noah (and possibly the Collective) from the Administrator's control. It's possible, however, that HABIT is simply lying; even HABIT admits this may be the case.
The Order frequently uses a unique symbol when depicting their beliefs, along with others, such as the Operator Symbol.
The Angel's Game
This unnamed symbol was used by the Dweller during the Angel's Game, as well as various others at times. It was also depicted on Baghead's mask, seemingly drawn in blood. Mr. Collector's mask also resembled this symbol. The symbol's exact purpose is unknown. It is written in text as ": >", sometimes accompanied by the phrase "listen with open eyes".