While the graph is titled "Hospital bed occupancy", the data is described as "COVID-19 Suspected + Positive Patients per CHHS Open Data Website"; ie it doesn't say anything about hospital bed occupancy.
The website has this to say about its data (my highlight):
PLEASE NOTE: This data may not match those compiled by local health departments, as this is a rapidly evolving situation meaning the counts are heavily dependent on when the data was pulled, as well as other factors. Timing of the data pulls for this dataset is stated in the data dictionary. Local health departments who post counts from later data pulls will have higher numbers for the same date. In addition, state cases are defined as laboratory-verified infections that have been submitted to the state surveillance system for reportable diseases, CalREDIE. Local Health Departments may have more expansive definitions of positive cases, or may have knowledge of cases that have not yet been entered in CalREDIE, which will also result in local health departments reporting higher case counts.
BusinessInsider describes Musks graph like this:
Musk tweeted a graph Wednesday, which he built using data from CHHS, to show that there are about 6,500 patients who have tested positive for COVID-19 occupying California hospital beds. There are nearly 75,000 hospital beds across the state. Musk accompanied his graph with the claim, "hospitals in California have been half empty this whole time."
So "half empty" doesn't seem to be a clear claim, but a figure of speech which isn't supported by the data Musk presented. I wasn't able to find specific data for his claim.
Note that Musks graph is highly misleading in other aspects as well. It compares positive COVID cases in hospitals at a given time vs the states prediction of accumulated cases in the general population. A quick search shows that there are as of now ~55000 confirmed cases and ~200.000 - 400.000 suspected cases in California. Both numbers are higher than the predictions in Musks graph.
More generally, there are some hospitals with a high percentage of empty beds, mostly because non-elect surgeries are canceled preemptively to deal with an expected increase in covid cases.