Something On Rock

[ReefTank327]

Yesterday I combined my 2.5 gallon nano tank with my 66 gallon reef. I looked over the live rock and everything looked good. However, once I put the rock in the tank I noticed something (which I couldn't identify) on the back of the rock. I attached an image. Please if anybody has any ideas what it couldbe, let me know.

[ReefTank327]

It is in the dark part under the rock. It is a red-brown color.

[Easy E]

It could be some type of encrusting sponge, which are generally not harmful.