Get the pool OUT!!
Remove the pool....in insurance parlance...it is an "attractive" nuisance and a release from a tenant with no insurable interest in the property is worthless. Give tenant 5 days - in writing - to remove the pool or remove it yourself. The grass will die underneath and you will continue to have problems with dirt under the pool because of oversplash of chemicals.....Advise tenant not to make alterations to the property in future w/o your written permission.
CPM * 45 years of property management experience * Age and experience ALWAYS trumps youth and enthusiasm