Be careful of falling.

If the context is caring for friends, you can say: be careful, don't slip.

When writing warning slogans, English slogans are imperative sentences. If the word "slip" is written as a key verb, it is easy for foreigners to misunderstand it as "please fall". Therefore, the key verb is not to slip, but to be careful

If indoors:

Beware of the wet floor.

Be careful of the uneven floor

If outside:

Watch out for wet ground/roads.

Beware of uneven ground.