Highly subjective/personal topic but I'll give you my take. And I'll caveat everything I say below in that I don't think
any guest should be expected to tip when staying CL. I suspect the large majority of guests do not.
It is true you cannot tip cash - that is not allowed in a big way and if you try a CM should refuse. I've read that it is a fireable offense but not sure if that's accurate.
On occasion I have given "gifts" such as a box of candy from the gift shop, a gift basket they can share, or similar gift. I've read about people making various things from home.
I do try to find something to do for the CMs at our repeat CLs like Poly where we've gotten to know some CMs that are there every time. In many ways they are an important part of our stay and one of the reasons we keep coming back, so I try to find some small way to show my appreciation.
The problem is it can be kind of awkward. You could run across a dozen or more CL CMs in a week long stay. There are typically multiple CMs at each food service plus the ones at the desks. Do you single out individuals? Give something large to share? That's where it gets a little fuzzy for me. Also why there's no one size fits all answer to your question.
That's my take. I guess either I'm too generous or a total scrooge - I've never really known!!
One "free tip" that's easy is to write Guest Services with the names of any CMs that you think provided great service and should be recognized. I make it a point to do that now when appropriate.
Not sure if any of that ever makes its way up/down the management line, but I'd like to think it does.