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While it could be from moisture condensing and running on your optics, most likely it is the grease in the baffle tube that has melted  (separated) and leaked inside your tube. 

All Celestron SCTs mount the primary mirror on a spindle that also acts as a baffle for light passing through the primary on its way to the eyepiece. This spindle is also part of the focusing mechanism for the scope. It’s greased to allow easy focusing movement.

Under rare combinations of storage, high ambient temperature and the wrong grease or too much grease, drops of oil will separate from the grease base and dribble inside and onto the tube mirrors and corrector plate.

This mess is best cleaned up by returning it to Celestron for factory service.

If you want to do it yourself, here’s an outline of how to do it. Remove the optical tube assembly from the forks. Completely disassemble the optical tube, including first removing the corrector plate -secondary mirror, then the front tube ring, the mirror cell, disassembling the focuser and lifting the primary mirror from the baffle spindle.

Wipe off heavy deposits. Use care when doing this with the optics. Degrease all the mechanical parts with a cleaner-degreaser (for example Loctite). Repaint the tube interior with flat black enamel paint. Clean the culprit grease off the baffle tube and spindle. Carefully clean the optics with a solvent (for example, acetone) or soap that will dissolve any remaining grease. Use caution not to damage coatings or the resin holding the mirror to the spindle.

Reapply a good grease (for example, Mobil One Premium) to the baffle.

Reassemble your optical tube. Remount the tube in the forks. Collimate your SCT optics.