Marks on Dendrobium leaves and stem? (Complete Newbie)

[Teapot]

Hello all!

Complete newbie to orchids so I apologise if this is a common or dumb sounding question; I have looked around online but I am still not sure if it is an actual problem or not.

I recently got given a "Dendrobrium Soft Cane". It has white and purple flowers and I only got given it a day or so ago. I have been looking up on how to care for it but I noticed when I got it that that one of the stems of the plant has marks on the leaves which the others do not.

The leaves seem to have white marks on the top side (they seem to rub off) and smaller white dots on the underside. The stems also seem to be going brown slightly where the leaves are attached. I have included photos of both and of the substrate the plant is in (Sorry if they are not the best quality!) in case it helps with identifying.

Thank you in advance! Again, like I said I am a complete newbie so I wanted to be completely sure if this is a problem or not in case I need to do something to fix it up.