I don't know what it is that you have but, I think it's aphids cause they secrete a sugary substance and I think that's what killed my phyllostachys nigra, the sugar substance attracts the ants and I could not get rid of it. The plant lost most of it's leaves and I thought it was going to make a comeback. Some of the leaves came back but it later died no shoots ever from this plant. I had the plant for two years, I think it had this when I bought it but not sure I was a newbie then.

I did a bit of research and think what I have is a kind of wax scale. Still trying to figure out what species, and how to control

Scale can be difficult to eliminate. Here's some info I found. Of course if your scale is a different species, this may not apply. With any scale, hit the crawlers.

Here is a link that might be useful: scale

As mentioned at the last link provided if you aren't going to watch for the crawlers and spray those then you are left with hand picking the adults and crushing them.

I took another look and I'm still not entirely certain what it is. It might be a kind of mealy bug but if so it produces a heavy waxy coating. That might just be their winter protection. I hope this didn't come from a nearby Miscanthus, which had a serious mealy bug problem, to the point that I finally just cut it down and kept cutting it until it died (too much trouble to dig it out!).