I’ve never had to deal with a mechanic’s lien but I’ve always been paranoid about them and have read up on them quite a bit. The system seems to favor the contractor over the homeowner, but the homeowner does have rights.

Start here, there’s lots of other resources through google:

http://www.cslb.ca.gov/consumers/legalissuesforconsumers/mechanicslien/IfMechanicsLienFiledAgainstYou.asp

I am not a lawyer or even a real-estate person, but IMO this pool guy really cannot file a legitimate lien against you. Of course they can still create some headache, like in any legal dispute.