Just happened to have been reading up on Cukes.
What kind have you got?
How long have you had it?
Have you noticed any obvious signs that it is eating? (ie: crap. ie: obvious sandy pellets).
Because you say it is "lazy" indicates to me that it has
not been eating and has actually been slowly dying of attrition, and the end is near. That white wispy stuff sounds like it could be holothurin, which is apparently toxic enough to wipe out your whole tank. I've never kept a Cuke before so this is by no means first hand experience here, but if I were in your shoes I'd get that cuke out of your tank now and do a few water changes also.
HTH and cheers
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