Learn Spanish (Latin America): Parece fácil, pero no lo es.

In Spanish, when you refer back to an adjective or a noun that describes what something is, you must use the neuter pronoun lo. Here, lo takes the place of the adjective fácil. You cannot just drop it and say pero no es because the verb ser needs that attribute to be represented.

Does lo change to la if the thing we are talking about is feminine?

No. When replacing an adjective or an abstract concept after the verb ser, the pronoun is always the invariable, neuter lo, regardless of the gender of the subject. Even if you were talking about a feminine noun like a test (la prueba), you would still say no lo es.

Could I use está instead of es since we are talking about how something seems right now?

No, because being easy (fácil) is considered an inherent characteristic of the task, not a temporary state or physical condition. Therefore, you must use es (from the verb ser) rather than está (from estar).

Could I say pero no lo parece instead of pero no lo es at the end?