That can/will happen if Wipe Free Space (or Drive Wiper) is inturrupted unexpectedly, such as by a power failure or unplugging an external drive while it is wiping.
Drive wipes work by filling up the 'free' parts of the disc with one or more files containing random data, (typically X's and Zeros) and then deleting that data.
That way any files that you have previously deleted get overwritten with random data, and so cannot later be recovered by recovery software.
Of course if anything should stop the drive wipe before it has deleted the random data that it has written then that random data will remain, taking up space on the disc.
That sounds like what has happened to you, the wipe has been unexpectedly halted and so couldn't remove the random data it had written.
So you have to remove it yourself now.
If you look in File Explorer then you should see one or more files (and/or a folder) with long, apparently random, names on the drive that you were wiping. They'll have a creation date of the wipe, and a big file size.
If you delete them the free space should be back.
Dunno why it's filled the disc like that; but it's possible that the Windows file table somehow thought that the size of the deleted files to be overwritten with random data was bigger than the remaining capacity of the disc?
That might happen if you deleted one or more (large) files and then put new files in the space that the deleted files had previously taken. - It shouldn't happen but it might.
eg. Windows wrongly tells CCleaner that a deleted file took up 1GB of space and so CCleaner wants to overwrite it with 1GB of random data, but there is no longer space to do that. (In your case above a deleted file larger than 581MB, which is all that's left on the disc).
In which case there would not be space on the disc for CCleaner to write what Windows said it needed to, and so Windows stops CCleaner before CCleaner gets the chance to erase the random data it had already written.
Unlike Nergal I don't think that running Wipe Free Space again will fix it automatically, because it isn't 'free space' anymore it's now got files containing random data in it.
(I could be wrong on that but don't think so).
PS. Once you have deleted the random files and got the space back then you might want to run 'System File Check' to check/fix the Windows system files.
https://support.microsoft.com/en-gb/help/929833/use-the-system-file-checker-tool-to-repair-missing-or-corrupted-system