As I read your question, I can sense you are new to Steemit. For a guide, your Steem Power will not decrease if you are going to upvote posts or comments in Musing, Steemit, or other dApps. Steem Power is vested Steem which influences the value of your upvote. What decreases when you are upvoting is your Voting Mana (formerly called Voting Power). Use your upvote wisely because once it goes down, it will take a lot of time to replenish back to 100% depending how much Votng Mana was consumed.
It will decrease you voting power, just the same as upvoting a post on Steemit will. Musing is on the Steem blockchain and thus a upvote on Musing is the same as on Steemit.
It WILL NOT decrease you Steem Power! Your Steem Power is locked into your account, and the only way to decrease it is to power down, or to delegate it to another account (and this is 'leased' and still owned by you, and can be recalled at any time)