Most likely it is white RTV silicon adhesive, used to prevent larger components from vibration.
Is this a problem?
I recently upgraded my old stereo system and was cleaning up the Sony STR AV720 receiver to give it to a relative. As I was cleaning inside, I noticed this white substance at the bottom of the output capacitors (wanted to attach photo but don't see a way to do that). It is like a paste or grease. Is that a problem (leak?) or is it normal? The unit seems to work fine. I don't want to give away equipment to a loved one that has a problem.